Use GitHub releases to download python versions

[MaksimZhukov]

This pull-request improves setup-python action to add ability to download specific version of Python on flight if it is not available by default.

Details:
setup-python action will download and install specific Python version from GitHub releases (actions/python-versions) in case the version is not found in the local cache. All versions of Python available for installation are published in actions/python-versions repository.
All available versions are listed in the version-manifest.json file.

Installation time:

• Ubuntu / macOS: 10-20 seconds
• Windows: ~ 1 minute (mostly related to fact that we use MSI installer for Python on Windows)

[konradpabjan]

Edit I can work on updating the README in a follow up PR. I'll add a few people as reviewers to double check everything

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Original comment

The README could use an update with regards to some of the new changes introduced here. A few things off the top of my head:

• An example with the new token input parameter
• An update to the available-versions-of-python section and hosted-tool-cache section with some clarifications around what is cached and what can be downloaded
• Some mention of our new fancy actions/python-versions repo 😀 (when should users file issues there vs here)

I also think some it would be good to have some information about what happens when a new version of Python is released (for example if 3.8.3 is released and replaces version 3.8.2). What can users expect to happen if they have 3.8 specified and what should they do if they need a specific version.

[bryanmacfarlane]

Note that concerning new releases, until the cache is update it will match against that first and we will have to be good about staying on top of it - ideally automated. In node tool in azure pipelines, we had an input which was always check source first (read through to dist). Can't do that with python since we build. Also note that an option like that says I must have the very latest at the cost of the instability

[konradpabjan]

Looks good to me! 👍

I've created an issue for some of the extra testing and updates that I'll take care of: https://github.com/github/c2c-actions-service/issues/1025

[konradpabjan]

I'll create a new tag and release tomorrow (v1.3.0). I'll also update the v1 tag at the same time.
Edit: We're going with a v2, expect a new tag, and a changelog as well. Our public docs should also get an update with v1 everywhere being bumped up to v2

There are a significant number of users pinned to @master so if there are any issues overnight we should hear about it. I don't expect anything to break or any compat issues since we've done a lot of testing.