Group Python Dependabot PRs too

To reduce the amount of PR rebasing that has to occur each month
when we update the Python package manager versions, given that
they need CHANGELOG.md entries each, which otherwise cause
conflicts.

See:
https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#groups--

GUS-W-22400243.

Author: edmorley

.github/dependabot.yml

@@ -33,3 +33,7 @@ updates:
     labels:
       - "dependencies"
       - "python"
+    groups:
+      # We don't limit grouping for Python to just minor/patch versions, since pip doesn't follow
+      # semver, plus having to update CHANGELOG.md means it's easier to just have a single PR.
+      python-dependencies: