Support a `.python-version` file

[indifferentghost]

Description:
The 2.5.0 release of actions/setup-node released support determine the node version via an .nvmrc file at the root. I think it'd be prudent to use the pyenv standard of a .python-version file to support this.

Justification:
It'd be nice to be DRY when specifying versions across multiple actions.

Are you willing to submit a PR?

[nikita-bykov]

Hello @indifferentghost, thanks for your feature request! We will look at it.

[indifferentghost]

Using a .*-version file is already defined by ruby/setup-ruby as well.

[fregante]

I'll append my request for Heroku’s runtime.txt format