feat: Added Backend 'EtsyOAuth2'

[amitray007]

Proposed changes

• Resolves Issue: https://github.com/python-social-auth/social-core/issues/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)
• [x] 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

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

Other information

• Added docs here: https://github.com/python-social-auth/social-docs/pull/199

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/python-social-auth/social-core/pull/874/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-social-auth) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/python-social-auth/social-core/pull/874/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-social-auth) | `77.70% <100.00%> (+0.11%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-social-auth#carryforward-flags-in-the-pull-request-comment) to find out more.

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[nijel]

Merged, thanks for your contribution!