Set .python-version to 3.10 for pyenv users (#4456)

The contribution docs for macOS say to install Python 3.10:


https://github.com/carbon-language/carbon-lang/blob/957599b2ab036d01b325aa6e82edea0fce7b4c53/docs/project/contribution_tools.md#L118-L124

For people who are using [pyenv](https://github.com/pyenv/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:


https://github.com/carbon-language/carbon-lang/blob/957599b2ab036d01b325aa6e82edea0fce7b4c53/docs/project/contribution_tools.md?plain=1#L162

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.

.pre-commit-config.yaml

@@ -213,6 +213,7 @@ repos:
           (?x)^(
               .bazelversion|
               .github/pull_request_template.md|
+              .python-version|
               compile_flags.txt|
               github_tools/requirements.txt|
               third_party/.*|

.python-version

@@ -0,0 +1 @@
+3.10