feat: Vendor OAuth1 from requests-oauthlib

[akx]

General

• Make sure you use semantic commit messages.
  Examples: "fix(google): Fixed foobar bug", "feat(accounts): Added foobar feature".
• All Python code must formatted using Black, and clean from pep8 and isort issues.
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• If your changes are significant, please update ChangeLog.rst.
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This PR vendors the single file used from requests_oauthlib to avoid an extra dependency on requests_oauthlib itself. The only changes made to the vendored file are mechanical (isort, black) and adding the ISC license header from requests-oauthlib.

[coveralls]

coverage: 95.567% (-0.1%) from 95.681%
when pulling c1eda02 on akx:vendor-requests-oauthlib
into 2e37568 on pennersr:main.

[pennersr]

Where you having problems with the existing dependency? Wondering what the advantage is of doing this, as requests-oauthlib seems to be well maintained.

[akx]

No particular problem, was just looking at dependency chains while looking at routine upgrades for projects.

Seems a waste to have one extra 1K LoC dependency for the single file that's being used from it.