feat: Added Backend 'EtsyOAuth2'

[amitray007]

Proposed changes

• Resolves Issue: [New Backend] Support Etsy OAuth2 #872
• Added a new backend: EtsyOAuth2
• Inherits BaseOAuth2PKCE for backend and OAuth2PkceS256Text for tests.

Types of changes

• Release (new release request)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (PEP8, lint, formatting, renaming, etc)
• Refactoring (no functional changes, no api changes)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build related changes (build process, tests runner, etc)
• Other (please describe):

Checklist

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added documentation to https://github.com/python-social-auth/social-docs

Other information

• Added docs here: docs: Added Etsy OAuth2 Documentation social-docs#199

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (34e4746) 77.59% compared to head (f14d699) 77.70%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #874      +/-   ##
==========================================
+ Coverage   77.59%   77.70%   +0.11%     
==========================================
  Files         343      345       +2     
  Lines       10521    10574      +53     
  Branches      699      699              
==========================================
+ Hits         8164     8217      +53     
  Misses       2200     2200              
  Partials      157      157              

Flag	Coverage Δ
unittests	77.70% <100.00%> (+0.11%)	⬆️

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[nijel]

Merged, thanks for your contribution!