fix: Use shutil.copytree over setuptools._distutils

[matthewfeickert]

Description

As setuptools is not the build backend there should be no reliance on setuptools or distutils for use. This exchanges setuptools._distutils.dir_util.copy_tree for shutil.copytree with dirs_exist_ok=True as pyhf supports Python 3.8+.

Follows up on an idea brought forward in PR #1711.

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• 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

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For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* As setuptools is not the build backend there should be no reliance on
  setuptools or distutils for use. This exchanges
  setuptools._distutils.dir_util.copy_tree for shutil.copytree with
  dirs_exist_ok=True as pyhf supports Python 3.8+.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.21%. Comparing base (e8789a2) to head (982171e).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2518   +/-   ##
=======================================
  Coverage   98.21%   98.21%           
=======================================
  Files          69       69           
  Lines        4543     4543           
  Branches      804      804           
=======================================
  Hits         4462     4462           
  Misses         48       48           
  Partials       33       33           

Flag	Coverage Δ
contrib	97.79% <ø> (ø)
doctest	98.08% <ø> (ø)
unittests-3.10	96.23% <ø> (ø)
unittests-3.11	96.23% <ø> (ø)
unittests-3.12	96.23% <ø> (ø)
unittests-3.8	96.25% <ø> (ø)
unittests-3.9	96.27% <ø> (ø)

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