util-docker: Re-add mypy to 20.04_all-dep Dockerfile

This was accidently removed in
https://gem5-review.googlesource.com/c/public/gem5/+/46999 due to a bad
rebase. mypy is needed by the pre-commit (Kokoro) tests.

Change-Id: Ice413e738520293ee51f0e8654d27d8fd88c103b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61089
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
1 file changed