Refactoring for personio-py 1.0

[klamann]

refactoring of personio-py. Planned changes:

• switch to poetry as build tool
• simplify the data model with pydantic or a similar library
• make use of Personio's new custom attributes API
• code cleanup & better test coverage

[codecov-commenter]

Codecov Report

Patch coverage: 89.1% and project coverage change: -0.6% ⚠️

Comparison is base (b5ead78) 87.3% compared to head (0181d55) 86.7%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #24     +/-   ##
========================================
- Coverage    87.3%   86.7%   -0.6%     
========================================
  Files           7       8      +1     
  Lines         946     860     -86     
========================================
- Hits          826     746     -80     
+ Misses        120     114      -6     

Files Changed	Coverage Δ
src/personio_py/g.py	77.7% <77.7%> (ø)
src/personio_py/client.py	80.7% <78.2%> (-1.1%)	⬇️
src/personio_py/util.py	83.3% <83.3%> (ø)
src/personio_py/models.py	91.9% <91.6%> (+1.4%)	⬆️
src/personio_py/__init__.py	100.0% <100.0%> (ø)
src/personio_py/errors.py	74.3% <100.0%> (ø)
src/personio_py/search.py	83.5% <100.0%> (+0.2%)	⬆️
src/personio_py/version.py	100.0% <100.0%> (ø)

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