Use nox for test automation

[bachya]

Describe what the PR does:

This PR introduces nox for test automation.

Does this fix a specific issue?

Fixes https://github.com/bachya/aioguardian/issues/

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md and docs/ with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #33 (3c121e5) into dev (012b2a6) will not change coverage.
The diff coverage is n/a.

❗ Current head 3c121e5 differs from pull request most recent head 8e8f44e. Consider uploading reports for the commit 8e8f44e to get more accurate results

@@            Coverage Diff            @@
##               dev       #33   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          178       178           
=========================================
  Hits           178       178           

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