Configure Dependabot to only update Python dependencies in the lockfile. (#19743)

See:
- https://github.com/element-hq/synapse/pull/19742
- https://github.com/element-hq/synapse/pull/19686

(etc)

Documentation
https://docs.github.com/en/code-security/reference/supply-chain-security/dependabot-options-reference#versioning-strategy--

We were considering `lockfile-only` but it sounds like
`increase-if-necessary` would increase the upper bound for us, if we had
one. Let's try it.

---------

Signed-off-by: Olivier 'reivilibre <oliverw@matrix.org>

.github/dependabot.yml

@@ -7,6 +7,7 @@ updates:
     package-ecosystem: "pip"
     directory: "/"
     open-pull-requests-limit: 10
+    versioning-strategy: "increase-if-necessary"
     schedule:
       interval: "weekly"
     # Group patch updates to packages together into a single PR, as they rarely