Defer to `install_requirement.py` in `beeware/.github`

[rmartin16]

Changes

• Removes install_requirement.py since it was copied in to beeware/.github for broader use
• This means it can no longer be used in tox.ini; ideally, this would be useful in tox.ini...but that likely means that the install_requirement.py needs to be incorporated in to a pip-installable package first.....we'll save this for a later day though...

PR Checklist:

• [x] All new features have been tested
• [X] All new features have been documented
• [X] I have read the CONTRIBUTING.md file
• [X] I will abide by the code of conduct

[rmartin16]

Once we've got this one merged, I'll rebase https://github.com/beeware/briefcase/pull/1843 on it and that should wrap up everything for "create package with attestation" PRs.

[rmartin16]

I just realized that if Tox is going to be building a package for these environment again, I should add in the optimizations for building wheels in tox. One second for another commit...