WIP: Support python nightly upload script

[aneeshusa]

Needed to fix servo/servo#16560.

Note in particular the first Salting on Windows: using Salt to install Python 3.
I haven't tested that at all, so I'd recommend rolling that out using the temporary Salt root.

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This change is 

[aneeshusa]

r? @larsbergstrom

[larsbergstrom]

This all looks good, code-wise - will do the test deployment stuff early next week when folks are around, in case I burn something down :-)

[aneeshusa]

The second commit will likely need to be updated to add the Python3 bits to the Windows $PATH in the build_windows_msvc var in buildbot/master/files/config/environment.py. I'm not sure exactly what the folder will be, so I'll wait until the test deployment to finalize that.

[metajack]

@larsbergstrom Do you want me to do this?

[larsbergstrom]

@metajack That would help! I'm all day in meetings through Friday afternoon, but will get to it then if you can't.

[metajack]

python3 package not found on windows

[aneeshusa]

I looked into this a little more and I don't think Python3 on Windows is the best direction for this, I will redo this with a different approach.

[metajack]

Doh. Do you have more details?

[aneeshusa]

A few reasons:

• Salt has support for installing Windows packages via MSIs, which actually works. However, it fails when checking to see if the installation succeeds - it either looks for the wrong key or in the wrong list of packages, I don't remember. Hopefully this is better in more up-to-date versions of Salt (we're over a year behind).
• Python3 on Windows doesn't install a python3 executable, only a python executable, which makes it trickier to make it available on the PATH. There is a py launcher that apparently can be used to invoke the right Python, but it seems a bit messy.

This may be worth revisiting down the road, but in the meantime I fixed up the servo PR to not requires saltfs changes.