Bump mkdocs-material version for security

[ajeklund]

Description:

Require mkdocs-material version >=9.5.5 for avoiding security vulnerability 64496. This is an alternative solution to PR #181.

Update: mkdocs-material v9.5.5 does not work with python 3.7. Therefore updating the tests "'pylint' & 'safety'" and "Build MkDocs Documentation" from python 3.7 to 3.9.

Type of change:

• Bug fix and code cleanup.
• New feature.
• Documentation update.
• Testing.

Checklist for the reviewer:

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.40%. Comparing base (0f65087) to head (7f8a32b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #186   +/-   ##
=======================================
  Coverage   77.40%   77.40%           
=======================================
  Files          17       17           
  Lines        1540     1540           
=======================================
  Hits         1192     1192           
  Misses        348      348           

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