Switch from runtime.txt to .python-version (#239)

Since use of the `runtime.txt` file is now deprecated in favour of
the `.python-version` file (since the latter is more widely supported
in the Python ecosystem).

This also switches to specifying only the major Python version
(which automatically uses the latest patch release), in order to:

1. Save us from having to bump the version in this repo every
   time a new Python patch version is released.
2. Ensure that anyone who closes this repo and uses it as the
   basis for their app is then always using an up to date patch
   version if they don't know/remember to bump the version
   themselves.

GUS-W-16971647.

Author: edmorley

.python-version

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