add tox generic and tox unit test step implementer

[itewk]

NOTE

replaces #219 with clean history

Purpose

Adding python tox generic and python tox unit test step implementer.

Breaking?

No

Integration Testing

Tested a local python tox test environment command execution to confirm the tests ran succesfully against the tox.ini configuration file.

• Tox generic makes no assumption about installed packages or commands defined in the tox.ini. It simply calls the target tox environment defined in the tox_env configuration parameter and executes the associated .ini config.
• The tox unit-test step implementer defaults to test for the tox_env parameter.

[codecov]

Codecov Report

Merging #223 (89c175c) into main (14ee488) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #223   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           84        87    +3     
  Lines         3479      3556   +77     
=========================================
+ Hits          3479      3556   +77     

Flag	Coverage Δ
pytests	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...tep_runner/step_implementers/shared/tox_generic.py	100.00% <100.00%> (ø)
...tep_runner/step_implementers/unit_test/__init__.py	100.00% <100.00%> (ø)
...tep_runner/step_implementers/unit_test/tox_test.py	100.00% <100.00%> (ø)
src/ploigos_step_runner/utils/tox.py	100.00% <100.00%> (ø)

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