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.