improve logging in shortcuts.py and turn to pytest in test_shortcut.py

[even-even]

Hi I've been using this project for my autotests for a very long time and I want to make a small contribution.

In this CC, the following was done:

1. improve error message in openapi_schema_validator/shortcuts.py
2. add more tests and turn from unittest to pytest in tests/unit/test_shortcut.py

Additionally done:

1. deleted unused libs in some files
2. corrected a typo in the documentation

I will be glad to receive feedback.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 83.04%. Comparing base (b326d47) to head (bc25733). Report is 85 commits behind head on master.

Files with missing lines	Patch %	Lines
openapi_schema_validator/shortcuts.py	40.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   84.38%   83.04%   -1.35%     
==========================================
  Files           6        6              
  Lines         237      230       -7     
  Branches       48       47       -1     
==========================================
- Hits          200      191       -9     
- Misses         24       26       +2     
  Partials       13       13              

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