feat(toolchain): Add new make vars for Python interpreter path compliant with --no_legacy_external_runfiles

[FrankPortman]

Using these new make vars in py_binary or py_test will correctly find the interpreter when setting --no_legacy_external_runfiles.

Fixes #2728

[aignas]

The change LGTM. I think the py2_runtime at some point should be dropped entirely.

Things that need to be done before this is merged:

• Document the PYTHON3_ROOTPATH make var somewhere, this file seems to be a good place to do that.
• Add :::versionchanged::: VERSION_NEXT_FEATURE line in the docstring for the rule, similar to how it is done elsewhere.
• Add a line in the CHANGELOG about the added feature.
• A test to ensure that this works is optional since we don't have any tests using the PYTHON3 var yet.

I might add these things if/when I have time myself, but if you want things to move quicker, please add these things.

[aignas]

This is perfect, thank you!