Support setting python_runtime stub_shebang in python repository

[fishcakez]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

A system installed python interpreter is required.

Issue Number: #691

What is the new behavior?

It is possible to avoid a system install python interpreter by setting hermetic_stub_shebang=True

Does this PR introduce a breaking change?

• Yes
• No

Other information

Extend the explanation (and added a warning) in ./README.md. A test is also added for every combination.

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[fishcakez]

For CI tests are failing because of unrelated issues fixed in #803 and #805.

[fishcakez]

Added bug report about the CI issues in #806. Will come back to this PR when the above PRs are merged.

[groodt]

Thanks for the PR. This isn't really a suitable solution to #691 however. Any solution would need to support Windows and it would also need to work when run outside the repository context (i.e. not using external/).

If you would like to proceed with this approach, you are able to do so already by overriding stub_shebang in the py_runtime.