validate_request and validate_response return value deprecation fix

[p1c2u]

Fixes #586

[codecov]

Codecov Report

Merging #589 (a01806d) into master (35872a9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head a01806d differs from pull request most recent head cec771c. Consider uploading reports for the commit cec771c to get more accurate results

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
+ Coverage   90.36%   90.38%   +0.01%     
==========================================
  Files         127      127              
  Lines        3738     3745       +7     
  Branches      454      454              
==========================================
+ Hits         3378     3385       +7     
  Misses        303      303              
  Partials       57       57              

Impacted Files	Coverage Δ
openapi_core/shortcuts.py	100.00% <100.00%> (ø)