Remove Python 2 compatibility code for Python 3.8+

[Copilot]

Summary

This PR removes all Python 2 backward compatibility code from the Beaker codebase, making it Python 3.8+ only. This resolves the issue where pyupgrade --py38-plus would break the code.

Changes Made

Core Compatibility Module (beaker/_compat.py)

• Completely rewritten to remove all Python 2 code
• Removed: PY2, JYTHON, PYVER, xrange_, add_metaclass, exec_, reraise
• Per reviewer feedback: Removed type aliases (NoneType, string_type, unicode_text, byte_string) - use native types directly
• Per reviewer feedback: Removed u_() function - use str directly

Files Updated to Use Native Types

• beaker/util.py: Use str, type(None) instead of aliases
• beaker/cache.py: Use str instead of unicode_text, str() instead of u_()
• beaker/session.py: Use str instead of string_type
• beaker/container.py: Removed unused unicode_text import
• beaker/converters.py: Use str instead of string_type
• beaker/ext/{mongodb,redisnm,redisclusternm}.py: Use str instead of string_type
• Test files: Replaced u_('...') with '...'

Other Changes

• setup.py: Added python_requires='>=3.8'
• cookie.py: Removed old Python version workarounds
• crypto/pbkdf2.py: Removed fallback PBKDF2 implementation
• container.py: Native metaclass= syntax

Test Results

• All core tests pass
• Fixed remaining u_() usages in tests/test_managers/base.py and tests/test_unicode_cache_keys.py that were causing CI failures

Original prompt

---

This section details on the original issue you should resolve

<issue_title>1.13.0: not ready for pyupgrade --py38-plus</issue_title>
<issue_description>Python 3.7 has been EOSed 27 Jun 2023 so I've been trying to update beaker code to python 3.8 using pyupgrade --py38-plus.
After apply patch generated by tis filter I found that pytest is failing

Here is pytest output:

+ PYTHONPATH=/home/tkloczko/rpmbuild/BUILDROOT/python-beaker-1.13.0-2.fc37.x86_64/usr/lib64/python3.10/site-packages:/home/tkloczko/rpmbuild/BUILDROOT/python-beaker-1.13.0-2.fc37.x86_64/usr/lib/python3.10/site-packages
+ /usr/bin/pytest -ra -m 'not network' --deselect tests/test_cache.py --deselect tests/test_cache_decorator.py --deselect tests/test_cachemanager.py --deselect tests/test_container.py --deselect tests/test_cookie_domain_only.py --deselect tests/test_cookie_expires.py --deselect tests/test_cookie_only.py --deselect tests/test_database.py --deselect tests/test_domain_setting.py --deselect tests/test_increment.py --deselect tests/test_managers/test_ext_mongodb.py --deselect tests/test_managers/test_ext_redis.py --deselect tests/test_managers/test_ext_rediscluster.py --deselect tests/test_memcached.py --deselect tests/test_session.py --deselect tests/test_sqla.py --deselect tests/test_unicode_cache_keys.py
3657536:C 23 Sep 2024 20:17:37.619 # WARNING Memory overcommit must be enabled! Without it, a background save or replication may fail under low memory condition. Being disabled, it can also cause failures without low memory condition, see https://github.com/jemalloc/jemalloc/issues/1328. To fix this issue add 'vm.overcommit_memory = 1' to /etc/sysctl.conf and then reboot or run the command 'sysctl vm.overcommit_memory=1' for this to take effect.
3657536:C 23 Sep 2024 20:17:37.619 * oO0OoO0OoO0Oo Redis is starting oO0OoO0OoO0Oo
3657536:C 23 Sep 2024 20:17:37.619 * Redis version=7.4.0, bits=64, commit=00000000, modified=0, pid=3657536, just started
3657536:C 23 Sep 2024 20:17:37.619 # Warning: no config file specified, using the default config. In order to specify a config file use redis-server /path/to/redis.conf
3657536:M 23 Sep 2024 20:17:37.620 * Increased maximum number of open files to 10032 (it was originally set to 1024).
3657536:M 23 Sep 2024 20:17:37.620 * monotonic clock: POSIX clock_gettime
3657536:M 23 Sep 2024 20:17:37.622 * Running mode=standalone, port=6379.
3657536:M 23 Sep 2024 20:17:37.622 # Warning: Could not create server TCP listening socket *:6379: bind: Address already in use
3657536:M 23 Sep 2024 20:17:37.622 # Failed listening on port 6379 (tcp), aborting.
============================= test session starts ==============================
platform linux -- Python 3.10.14, pytest-8.2.2, pluggy-1.5.0
rootdir: /home/tkloczko/rpmbuild/BUILD/beaker-1.13.0
configfile: setup.cfg
collected 19 items / 17 errors

==================================== ERRORS ====================================
_____________________ ERROR collecting tests/test_cache.py _____________________
/usr/lib/python3.10/site-packages/_pytest/python.py:492: in importtestmodule
    mod = import_path(
/usr/lib/python3.10/site-packages/_pytest/pathlib.py:591: in import_path
    importlib.import_module(module_name)
/usr/lib64/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1050: in _gcd_import
    ???
<frozen importlib._bootstrap>:1027: in _find_and_load
    ???
<frozen importlib._bootstrap>:1006: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:688: in _load_unlocked
    ???
/usr/lib/python3.10/site-packages/_pytest/assertion/rewrite.py:178: in exec_module
    exec(co, module.__dict__)
tests/test_cache.py:9: in <module>
    from beaker.middleware import CacheMiddleware
beaker/middleware.py:11: in <module>
    from beaker.cache import CacheManager
beaker/cache.py:20: in <module>
    import beaker.ext.database as database
beaker/ext/database.py:11: in <module>
    from beaker.ext.sqla import SqlaNamespaceManager
E     File "/home/tkloczko/rpmbuild/BUILD/beaker-1.13.0/beaker/ext/sqla.py", line 84
E       log.debug OSError
E                 ^^^^^^^
E   SyntaxError: invalid syntax
________________ ERROR collecting tests/test_cache_decorator.py ________________
/usr/lib/python3.10/site-packages/_pytest/python.py:492: in importtestmodule
    mod = import_path(
/usr/lib/python3.10/site-packages/_pytest/pathlib.py:591: in import_path
    importlib.import_module(module_name)
/usr/lib64/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
<frozen importlib._bootstrap>:1050: in _gcd_import
    ???
<frozen importlib._bootstrap>:1027: in _find_and_load
    ???
<frozen importlib._bootstrap>:1006: in _find_and_load_unlocked
    ???
<frozen importlib._bootstrap>:68...

</details>

- Fixes bbangert/beaker#244

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