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.

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