feat: Drop Python 3.7 support

[matthewfeickert]

Description

Following Discussion #2042, drop Python 3.7 support to follow NEP 29 development recommendations. As a result, drop the requirement of typing_extensions as that is needed for Python 3.7 only and update the lower bound on SciPy to v1.3.2 — the first release with Python 3.8 wheels.

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

* Drop Python 3.7 support to follow NEP 29 development recommendations.
   - c.f. https://github.com/scikit-hep/pyhf/discussions/2042
* Drop typing_extensions as a requirement as that is needed by Python 3.7 only.
* Update lower bound of the supported scipy versions to v1.3.2, the first scipy release
  with Python 3.8 wheels.
* Update pre-commit hooks to use Python 3.8.

[codecov]

Codecov Report

Base: 98.30% // Head: 98.30% // No change to project coverage 👍

Coverage data is based on head (031c9c0) compared to base (3042b27).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2044   +/-   ##
=======================================
  Coverage   98.30%   98.30%           
=======================================
  Files          69       69           
  Lines        4530     4530           
  Branches      645      645           
=======================================
  Hits         4453     4453           
  Misses         45       45           
  Partials       32       32           

Flag	Coverage Δ
contrib	27.52% <ø> (ø)
doctest	61.14% <ø> (ø)
unittests-3.10	96.31% <ø> (ø)
unittests-3.7	?
unittests-3.8	96.33% <ø> (ø)
unittests-3.9	96.35% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pyhf/contrib/utils.py	100.00% <ø> (ø)

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