build: migrate to GitHub Actions [BB-7150]

[Agrendalath]

This:

1. Adds the suppport for bleach 6.
2. Adds GitHub workflows.
3. Adds a template for PRs.
4. Modernizes setup.py and moves the package version to __init__.py.
5. Adds pip-tools to logically organize dependencies and allow scheduled requirements updates.
6. Adds tox to simplify the testing setup.

Review instructions

It will be easier to review this commit by commit.

Testing instructions

1. Check that the CI is passing.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@0e653d7). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #25   +/-   ##
=========================================
  Coverage          ?   62.65%           
=========================================
  Files             ?        4           
  Lines             ?      166           
  Branches          ?       15           
=========================================
  Hits              ?      104           
  Misses            ?       61           
  Partials          ?        1           

Flag	Coverage Δ
unittests	62.65% <0.00%> (?)

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[navinkarkera]

👍

• I tested this: (CI is passing)
• I read through the code
• I checked for accessibility issues
• Includes documentation
• I made sure any change in configuration variables is reflected in the corresponding client's configuration-secure repository.