[FIX] core: vendored sessions.py so it actually works
[FIX] core: vendored sessions.py so it actually works

* remove SessionMiddleware we don't need as it's responsible for most
  imports
* fix relative imports to absolute imports from werkzeug
* remove py2/py3 compatibility imports, shims and conditions as we're
  P3 only
* remove deprecation warning (duh)
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