Make typing_extensions conditional to Python < 3.8

[mgorny]

Pull Request check-list

Please make sure to review and check all of these items:

• Does $ tox pass with this change (including linting)?
• Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• Is the new or changed code fully tested?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

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Description of change

All the typing features used by redis-py are available in Python 3.8+,
so typing_extensions is not used at all in newer versions of Python.
Adjust the dependencies accordingly.

[codecov-commenter]

Codecov Report

Merging #2062 (d095608) into master (ef4caf5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2062   +/-   ##
=======================================
  Coverage   91.20%   91.20%           
=======================================
  Files         100      100           
  Lines       21319    21319           
=======================================
  Hits        19443    19443           
  Misses       1876     1876           

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)

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[mgorny]

I'm getting some test failures from tox but they seem irrelevant to the changes at hand.

[chayim]

Thanks @mgorny for this!

[mgorny]

Thanks!