Remove filtered warning after fixed dependency released

[hugovk]

The certifi library has just been released with its deprecation warning fixed, we can remove this workaround:

• https://github.com/certifi/python-certifi/pull/199#issuecomment-1241602032

We need to keep the gidgethub workaround, it's fixed but not yet released:

• https://github.com/brettcannon/gidgethub/pull/185

[Mariatta]

I'll cut a release of gidgethub today

[Mariatta]

gidgethub has just been released, so I think it means we can remove both now.

[hugovk]

Updated to remove the other one now! 🚀