Python2 support drop

[p1c2u]

Backward incompatibilities

• Python2 support drop

Fixes #331

[codecov]

Codecov Report

Merging #335 (87c9f85) into master (23a8b57) will decrease coverage by 0.83%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #335      +/-   ##
==========================================
- Coverage   96.65%   95.82%   -0.84%     
==========================================
  Files          80       79       -1     
  Lines        1643     1627      -16     
==========================================
- Hits         1588     1559      -29     
- Misses         55       68      +13     

Impacted Files	Coverage Δ
openapi_core/casting/schemas/util.py	100.00% <100.00%> (ø)
openapi_core/contrib/django/requests.py	100.00% <100.00%> (ø)
openapi_core/contrib/flask/requests.py	93.33% <100.00%> (ø)
openapi_core/contrib/requests/requests.py	100.00% <100.00%> (ø)
openapi_core/deserializing/media_types/util.py	100.00% <100.00%> (ø)
openapi_core/schema/schemas.py	100.00% <100.00%> (ø)
openapi_core/schema/servers.py	100.00% <100.00%> (ø)
openapi_core/templating/paths/finders.py	100.00% <100.00%> (ø)
openapi_core/testing/requests.py	100.00% <100.00%> (ø)
...penapi_core/unmarshalling/schemas/unmarshallers.py	94.82% <100.00%> (-0.06%)	⬇️
... and 4 more

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