Add an example Python Support case study (500USD bounty)

[lihaoyi]

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From the maintainer Li Haoyi: I'm putting a 500USD bounty on this issue, payable by bank transfer on a merged PR implementing this.

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This would be similar to the example Typescript support case study we have in https://mill-build.org/mill/0.12.1/extending/new-language.html, but instead of typescript/npm/tsc/rollup we would use python/pip/mypy/pex (or equivalent). We should take care to make sure things are installed in the dest folders using virtualenvs or whatever, rather than globally as python traditionally defaults to

Doesn't need to be production ready, just complete enough for educational purposes illustrating the various Mill features

[himanshumahajan138]

@lihaoyi Sir, i have a great knowledge of python and now for mill too (after your guidance and merged pr's) i would like to work around this i know bounties are open until merged pr but i just want to get a safe part from clash so i commented

Hope you understand !

[c0d33ngr]

@lihaoyi wouldn't it be better structure like this

newlang/
  typescript/
    ...
  python3/
    ...

[lihaoyi]

@c0d33ngr doesn't really matter, we can shuffle folders around later. The important thing is the examples and explanations