Migrate BitbucketServer to oauthlib

[Swatinem]

This pretty much copied over the oauth-related code from Bitbucket to BitbucketServer, and gets rid of the oauth2 and tlslite-ng dependencies.

This does a few simplifications compared to the Bitbucket code, namely not caring about non-json bodys, as I believe the existing code will only ever pass JSON payloads.

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fixes part of https://github.com/codecov/internal-issues/issues/436
prerequisite for #423

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.82%. Comparing base (bb18502) to head (7f6ddd9).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #426      +/-   ##
==========================================
- Coverage   90.23%   89.82%   -0.41%     
==========================================
  Files         389      324      -65     
  Lines       11792     9383    -2409     
  Branches     2063     1676     -387     
==========================================
- Hits        10640     8428    -2212     
+ Misses       1052      891     -161     
+ Partials      100       64      -36     

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[michelletran-sentry]

I'm OK with this, but would prefer to remove this altogether if it's not being used.

[Swatinem]

I'm OK with this, but would prefer to remove this altogether if it's not being used.

I will merge as-is then, following up on conversations to completely remove this.

Summary so far is that this is indeed not enabled in SaaS, so self-hosted might be the only ones impacted by this, and it seems like BitbucketServer itself is already EOL.