Update `cla-bot` entry after moving it to the `python` org

[ezio-melotti]

As a follow-up to #469 (and #470), this is a reminder to update the cla-bot entry in the README after moving it to the python org (currently the repo is at ambv/cla-bot).

Merge this after the bot has been migrated.

[hugovk]

(Converting to draft whilst awaiting migration.)

[ezio-melotti]

@ambv, is the migration of the cla-bot to the python org still planned?

[CAM-Gerlach]

Seems like it should be, as per the Python-org repo policy, the org is for key infrastructure needed for CPython development, and it seems like a reasonable bus factor issue for the bot to be developed under any one particular core dev's personal account...