gitlab-ci: use mypy from Debian/bullseye to prevent random breakage
gitlab-ci: use mypy from Debian/bullseye to prevent random breakage

mypy 0.901 was released and everything broke:
https://gitlab.com/linsui/fdroidserver/-/jobs/1330206567

My point is to reduce the number of false job fails like this one. We
have a lot of checkers running, I really think they need to prove they are
adding value before we invest any time maintaining them. mypy is still
"wait and see" in terms of the adding any value.

!951

fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver