Add Django 2.2 wrapper

[plambein]

I simply added a wrapper for Django 2.2 requests and responses. Not sure if and where I should add integration tests, feedback is welcome.

[codecov]

Codecov Report

Merging #162 into master will decrease coverage by 2.03%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
- Coverage   95.94%   93.91%   -2.04%     
==========================================
  Files          62       63       +1     
  Lines        1702     1742      +40     
==========================================
+ Hits         1633     1636       +3     
- Misses         69      106      +37

Impacted Files	Coverage Δ
openapi_core/wrappers/django.py	0% <0%> (ø)
openapi_core/schema/schemas/_format.py	90.9% <0%> (+3.89%)	⬆️

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[p1c2u]

@plambein Thank you for the contribution. This is great. I will take a closer look and figure out how to cover it with tests.

[p1c2u]

I rebased and reworked the branch with new factories #167 .Closing this one.