Add arun.gov.uk source

[whi-tw]

BS4 parser required as Arun doesn't support ICS.

Have to set up a session initially by sending GET requests to two URLs to:

1. Start the session
2. Activate the session

This is quite brittle, but I guess so are most of these form scrapers!

[5ila5]

Thanks for your contribution

Did some minor reformatting (.pre-commit-config.yaml)

[whi-tw]

Makes perfect sense - sorry, I didn't switch it on apparently. nor did I sign my commit. what a scrub! Thanks for just sorting it :)