make jsonschema4 compatible (DeprecationWarning fix) reintroduced

[p1c2u]

Reintroduces #25 after revert #34

This reverts commit 6ae997f, reversing
changes made to 22c5892.

[codecov]

Codecov Report

Merging #38 (4d76c99) into master (97970e2) will increase coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   67.02%   67.19%   +0.17%     
==========================================
  Files           6        6              
  Lines         188      189       +1     
  Branches       40       40              
==========================================
+ Hits          126      127       +1     
  Misses         50       50              
  Partials       12       12              

Impacted Files	Coverage Δ
openapi_schema_validator/validators.py	95.45% <100.00%> (+0.21%)	⬆️

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