build: Update lower bound of typing-extensions to v3.7.4.3

[matthewfeickert]

Description

Update lower bound of the supported typing-extensions versions to v3.7.4.3 to ensure SupportsIndex is available, which is used as of PR #1940.

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

* Update lower bound of the supported typing-extensions versions to v3.7.4.3
  to ensure SupportsIndex is available, which is used as of PR #1940.
   - Amends PR #1940.
* Update tests/constraints.txt to use typing-extensions==3.7.4.3.

[matthewfeickert]

PR #1962 is a follow up PR to this to deal with the fact that numpy.typing was introduced in numpy v1.20.0.

[codecov]

Codecov Report

Merging #1961 (467116a) into master (fc9ffd1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1961   +/-   ##
=======================================
  Coverage   98.24%   98.24%           
=======================================
  Files          68       68           
  Lines        4380     4380           
  Branches      728      728           
=======================================
  Hits         4303     4303           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	26.55% <ø> (ø)
doctest	60.54% <ø> (ø)
unittests-3.10	96.14% <ø> (ø)
unittests-3.7	96.12% <ø> (ø)
unittests-3.8	96.16% <ø> (ø)
unittests-3.9	96.18% <ø> (ø)

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