Pin local Python version to lowest supported (#351)

Just helps to ensure when developing that we're targeting the lowest
supported version, otherwise it'll only be caught in CI.

Ref:
https://docs.astral.sh/uv/concepts/python-versions/#python-version-files

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",