Set .python-version to 3.10 for pyenv users

[samestep]

The contribution docs for macOS say to install Python 3.10:

carbon-lang/docs/project/contribution_tools.md

Lines 118 to 124 in 957599b

	# Install Homebrew tools.
	brew install \
	bazelisk \
	gh \
	llvm \
	[email protected] \
	pre-commit

For people who are using pyenv instead of Homebrew Python, this PR adds a .python-version file to specify Python 3.10 for pyenv to use.

However, I also see that later in the same document, the docs only say that Python >=3.9 is required:

carbon-lang/docs/project/contribution_tools.md

Line 162 in 957599b

- Carbon requires Python 3.9 or newer.

So in that case, feel free to just close this PR. I also saw that #778 specifically moved Carbon away from pyenv, so if pyenv is discouraged in general, also feel free to just close this.

[jonmeow]

LG -- Sorry, for some reason I'd thought someone else got this PR.

[jonmeow]

Sorry, I see the pre-commit issue. In .pre-commit-config.yml, around line 215, I'd suggest inserting an exception, basically:

               .github/pull_request_template.md|
+              .python-version|
               compile_flags.txt|

[samestep]

I just added the copyright text instead, guessing that works too?

[jonmeow]

I just added the copyright text instead, guessing that works too?

See the error -- Python's strict about file contents.

[samestep]

Huh, weird... I guess actions/setup-python is buggy, since pyenv says this should be fine:

You can also specify multiple versions in a .python-version file by hand, separated by newlines. Lines starting with a # are ignored.

In any case, I've now implemented your suggestion. Sorry for taking up so much of your time on this!

[jonmeow]

In any case, I've now implemented your suggestion. Sorry for taking up so much of your time on this!

No problem, it's really not much time and I should've reviewed last week. FWIW, I wasn't actually looking at what should be supported.