python-common: add a dedicated tox env to run mypy

python-common: add a dedicated tox env to run mypy

IMO it's not a good practice to overload a tox rule with multiple
different test tools. It forces the tools to share the same virtualenvs
and makes it impossible to run the tools individually. A separate mypy
env also better matches the other tox.ini files in the ceph tree.
Since the new 'mypy' env is in the default env list it will continue
to get run automatically when no specific envs are selected.

Signed-off-by: John Mulligan <jmulligan@redhat.com>