Use fork URLs/versions

[AndrewFerr]

This has a few practical benefits:

• The version bot command will link to this fork instead of to upstream. (Links work without this, but they give a warning of This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.)
• The __version__ property of the Python package will represent our fork's version, which may help for scripting & debugging.

[AndrewFerr]

Snuck this in with https://github.com/vector-im/mautrix-telegram/pull/59 (except for the change to __version__ which was bad anyways)