Switch from Dependabot to `update-dependencies` action for Python dependencies

[StevenMaude]

Dependabot continues to give us problems for Python in various repositories, with an assortment of problems.

We could move our Python updates to our own update-dependencies action, as we have for job-server.

[nishtha-kalra]

The workflow started failing after merging because the app-id and private-key added in this PR is associated with opensafley-core, not opensafely.
There isn't an app for the opensafely organisation, and Peter plans to create a separate app per organisation. He will create one next week. Therefore this is blocked as of now.
Related slack thread https://bennettoxford.slack.com/archives/C63UXGB8E/p1787754197423929
I have reverted the previous change so that the workflow stops failing and python updates are shifted back to dependabot.

[StevenMaude]

I think this should be unblocked now.