Get rid of unmarshal in schema

[p1c2u]

No description provided.

[codecov]

Codecov Report

Merging #189 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
+ Coverage   96.27%   96.35%   +0.08%     
==========================================
  Files          97       97              
  Lines        1958     1948      -10     
==========================================
- Hits         1885     1877       -8     
+ Misses         73       71       -2

Impacted Files	Coverage Δ
openapi_core/schema/schemas/models.py	98.52% <ø> (+1.12%)	⬆️
openapi_core/unmarshalling/schemas/exceptions.py	92.59% <ø> (+1.41%)	⬆️
openapi_core/validation/request/validators.py	95.32% <100%> (+0.13%)	⬆️
openapi_core/validation/response/validators.py	89.85% <100%> (+0.46%)	⬆️

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