Python interpreter can now be overridden

[apt-itude]

In case the Python runtime used to build a par is not exactly the same path as the target interpreter, this allows the interpreter to be explicitly set instead of inferred from the stub file.

Example:

par_binary(
    name = "mybinary",
    srcs = ["mybinary.py"],
    compiler_args = ["--interpreter", "/usr/bin/env python3.6"],
)

[googlebot]

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📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

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[apt-itude]

I signed it!

(alex@128technology.com should now be a member of opensource@128technology.com)

[duggelz]

@apt-itude Unfortunately I no longer work for Google, and don't have commit access to this repo any longer. Change looks good to me. A test case would be nice, though.

[tmc]

@apt-itude this adds a basic test: tmc@4db9b96

[brandjon]

tmc@ expressed an interest in forking this PR. Regarding the CLA situation: I believe the CLA bot hasn't updated this PR because no new commit has been pushed, but I can verify that alex@128technology.com is covered by the 128 Technology CLA.

[apt-itude]

Hey guys, sorry I haven't been on top of this. I'll push a new commit with that test case.

[googlebot]

CLAs look good, thanks!

[brandjon]

Thanks for the PR!