Allow customizing Python stub script's shebang in the Python toolchain

[brandjon]

Currently the stub script requires (via its shebang) that the target system have a python command available on PATH. If the target is PY3, the user might reasonable expect that it work if python is not available but python3 is. We can modify the shebang in this case to be #!/usr/bin/env python3. However doing so might break if the system has a (Python 3) python command but not a python3 command.

See #8665 for a case where the shebang has been a problem in practice.

[brandjon]

More generally, we can support customizing the shebang as part of the Python toolchain. I'm not clear on how useful this feature would be in the short to medium term. Long term we'd like to eliminate the Python stub altogether and replace it with a C++ one.

[brandjon]

Repurposing this bug to track the FR to customize the shebang string in the toolchain. That supersedes requests to make it work with a particular interpreter version / system environment xyz. See also #11269.

@brandjon I'm not sure if the Bazel maintainers prefer to resolve this independent of issues like #8446 or #137, but I'm pursuing this internally and would be happy to generate a PR within the next day or two.

• Any naming suggestions for the new py_runtime parameter?
• Are PyRuntimeInfoTest test cases sufficient, or do I need to get down and dirty with python_stub_test.sh?