feat: Add support for Python 3.9 across all backends

[matthewfeickert]

Description

• Resolves Support Python 3.9 #1515
• Requires PR fix: Ensure TensorFlow backend Poisson compatibility with other backends #1001

pyhf is already Python 3.9 compliant, with the exception of TensorFlow. With PR #1000, this will be resolved and all pyhf backends can be tested against Python 3.9.

.github/workflows/release_tests.yml won't get updated yet though as there is no Python 3.9 release out to test against.

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

* Add official support for Python 3.9 and add Python 3.9 to CI
   - Required PR #1001 to get Python 3.9 support of TensorFlow and TensorFlow Probability
* Add Python 3.9 support to PyPI metadata
* Update GitHub Actions workflows to use Python 3.9
   - .github/workflows/release_tests.yml will be updated after pyhf v0.6.3 is released
* Update Dockerfile to use python:3.9-slim-bullseye as base image

[codecov]

Codecov Report

Merging #1574 (5b013c5) into master (5247c16) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1574   +/-   ##
=======================================
  Coverage   97.70%   97.70%           
=======================================
  Files          63       63           
  Lines        4050     4050           
  Branches      576      576           
=======================================
  Hits         3957     3957           
  Misses         54       54           
  Partials       39       39           

Flag	Coverage Δ
contrib	25.43% <ø> (ø)
unittests	97.48% <ø> (ø)

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[matthewfeickert]

.github/workflows/release_tests.yml should be updated once v0.6.3 is out and there is a Python 3.9 release to test.