build: Set lower bound of typing-extensions to v3.7.4

[matthewfeickert]

Description

Add a lower bound of the supported typing_extensions version to v3.7.4 as TypedDict was added in v3.7.4.

The typing_extensions module contains both backports of these changes as well as experimental types that will eventually be added to the typing module, such as Protocol or TypedDict.

• c.f. https://pypi.org/project/typing-extensions/3.7.4/

Amends PR #1934.

Example:

(venv) $ python -m pip --quiet install 'typing-extensions==3.7.2' && python -c 'from typing_extensions import Literal, TypedDict' && echo $?
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: cannot import name 'TypedDict' from 'typing_extensions' (/venv/lib/python3.7/site-packages/typing_extensions.py)
(venv) $ python -m pip --quiet install 'typing-extensions==3.7.4' && python -c 'from typing_extensions import Literal, TypedDict' && echo $?
0

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Set the lower bound of the supported typing_extensions versions to v3.7.4
  as TypedDict was added in v3.7.4.

  > The typing_extensions module contains both backports of these changes as
  > well as experimental types that will eventually be added to the typing module,
  > such as Protocol or TypedDict.
   - c.f. https://pypi.org/project/typing-extensions/3.7.4/

   - Amends PR #1934
* Add typing-extensions==3.7.4 to tests/constraints.txt.

[codecov]

Codecov Report

Merging #1938 (46ace0d) into master (dcbd9ac) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1938   +/-   ##
=======================================
  Coverage   98.26%   98.26%           
=======================================
  Files          69       69           
  Lines        4439     4439           
  Branches      748      748           
=======================================
  Hits         4362     4362           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	25.81% <ø> (ø)
doctest	61.02% <ø> (ø)
unittests-3.10	96.14% <ø> (ø)
unittests-3.7	96.13% <ø> (ø)
unittests-3.8	96.17% <ø> (ø)
unittests-3.9	96.19% <ø> (ø)

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[matthewfeickert]

Given that the minimum supported dependencies workflow has tests/test_import.py passing, then this should be good to go.