Reapply "Pin local Python version to lowest supported (#351)" (#375)

This reverts commit fb15875.

When working through #384 without a pin set, running `make` would
default to Python 3.13 locally. (Not sure if this is a `uv` version
thing, or what.)

I think it's best to keep this pinned in Python libraries to the lowest
supported Python version to avoid confusion, especially since we don't
specify a `uv` version to use (for now).

Author: stefanvanburen

.python-version

@@ -0,0 +1 @@
+3.9

pyproject.toml

@@ -9,6 +9,7 @@ readme = "README.md"
 license = "Apache-2.0"
 license-files = ["LICENSE"]
 keywords = ["validate", "protobuf", "protocol buffer"]
+# Keep our minimum version synced with ./.python-version.
 requires-python = ">=3.9"
 classifiers = [
   "Programming Language :: Python :: 3.9",