Enforce python3 and deprecate the use of python2

[henryboisdequin]

This PR enforces python3 and depreciates the use of python2 to build the Rust compiler. As python2 is no longer supported and we are approaching python4, we should move towards python3. Python2 is no longer maintained and could bring bugs to rustc if used. Even though Rust is about compatibility, the Python team highly recommends deprecating python2 since it's no longer maintained and security/general bug reports will not be looked at. Additionally, if rustc wanted to use python3 only features/python3 only packages, this would break when using python2.

Note: this PR also formats the Python code

From the Python "Sunset Python 2" blog post:

What will happen if I did not upgrade by January 1st, 2020?

If people find catastrophic security problems in Python 2, or in software written in Python 2, then most volunteers will not help fix them. If you need help with Python 2 software, then many volunteers will not help you, and over time fewer and fewer volunteers will be able to help you. You will lose chances to use good tools because they will only run on Python 3, and you will slow down people who depend on you and work with you.
Some of these problems started on January 1. Other problems will grow over time.

[rust-highfive]

r? @Mark-Simulacrum

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[jyn514]

Please see #71818 (comment); I don't think this is the sort of change that should be made with a PR out of the blue.

[henryboisdequin]

@jyn514 I agree, I didn't see that comment. I will close this PR and follow up on Zulip.