[MINOR][INFRA] Exclude .python-version from git

### What changes were proposed in this pull request?

[pyenv](https://github.com/pyenv/pyenv) is a popular tool for managing multiple versions of Python.

> The first `.python-version` file found (if any) by searching each parent directory, until reaching the root of your filesystem.

### Why are the changes needed?

Improve developer experience who use pyenv to manage Python environments.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#47939 from pan3793/minor-pyenv.

Authored-by: Cheng Pan <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

.gitignore

@@ -8,6 +8,7 @@
 *.swp
 *~
 .java-version
+.python-version
 .DS_Store
 .ammonite
 .bloop