Django2 support drop

[p1c2u]

• Django/DRF project integration tests
• Added DjangoOpenAPIMiddleware middleware

[codecov]

Codecov Report

Merging #358 (7817a64) into master (972c975) will increase coverage by 0.61%.
The diff coverage is 96.70%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
+ Coverage   96.04%   96.66%   +0.61%     
==========================================
  Files          80       80              
  Lines        1696     1739      +43     
==========================================
+ Hits         1629     1681      +52     
+ Misses         67       58       -9     

Impacted Files	Coverage Δ
openapi_core/contrib/django/middlewares.py	92.30% <92.30%> (ø)
openapi_core/contrib/django/__init__.py	100.00% <100.00%> (ø)
openapi_core/contrib/django/handlers.py	100.00% <100.00%> (ø)
openapi_core/contrib/django/requests.py	100.00% <100.00%> (ø)
openapi_core/contrib/django/responses.py	100.00% <100.00%> (ø)

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