Assume that typing_extensions is always installed.

[thetorpedodog]

Changes

Because we strictly depend upon typing_extensions both explicitly and implicitly, we don't need the code which checks if it isn't installed.

(This is a precursor to work on #479)

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• [n/a] You've added tests (in tests/) added which would fail without your patch
• [n/a] You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• [n/a] You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

[coveralls]

coverage: 94.341% (+0.2%) from 94.139%
when pulling e23091a on thetorpedodog:always-typing-extensions
into 016f813 on agronholm:master.