Add .python-version

Since specifying an explicit Python version is recommended for determinism.

Adding the `.python-version` file will suppress the warning otherwise shown during the build, and prevent the version changing when 3.14 is made the default at the end of this year.

The current default Python version is 3.13, so the actual version used remains the same.

See also:
https://devcenter.heroku.com/articles/python-support#specify-a-python-version
https://devcenter.heroku.com/articles/python-runtimes

Author: edmorley

.python-version

@@ -0,0 +1 @@
+3.13