Ignore PEP 496 environment markers in requirements.txt

[joshfriend]

I used poetry to export my requirements.txt file and it adds stuff to the package version like this:

flask==1.1.2; (python_version >= "2.7" and python_full_version < "3.0.0") or (python_full_version >= "3.5.0")
gevent==21.1.2; python_version >= "3.4" and python_full_version < "3.0.0" or python_version >= "3.4" and python_full_version >= "3.5.0"
greenlet==1.0.0; python_version >= "3.4" and python_full_version < "3.0.0" and platform_python_implementation == "CPython" or python_version >= "3.4" and python_full_version >= "3.5.0" and platform_python_implementation == "CPython"
gunicorn==20.0.4; python_version >= "3.4"

which makes the tool fail with fun errors like [ERROR] Failed to gather gevent (URL can't contain control characters. '/pypi/gevent/21.1.2; python_version >= "3.4" and python_full_version < "3.0.0" or python_version >= "3.4" and python_full_version >= "3.5.0"/json' (found at least ' '))