mgr/vol: fix flake8 warnings

mgr/vol: fix flake8 warnings

Fix warnings printed by the command "flake8 --select=F,E9
--exclude=venv,.tox src/pybind/mgr/volumes/".

While working with mgr/vol, syntax errors are not printed anywhere. Any
attempt to run vstart.sh after such faulty patch causes vstart.sh to go
in a infinite loop. And running "ceph fs volume" command prints "no such
command exists". This doesn't tell the actual issue and causes
confusion.

When flake8 is run, the issue is not immediately apparent due to so many
warnings. Therefore, fix these warnings so that it becomes easier to
spot such critical issues here onwards.

Note: mypy uses comments like "#type: Dict" for type checking and
therefore counts 'from typechecking import Dict' as not unsued. But
flake8 doesn't recognize type hints embedded in comment. Therfore,
switch to actually using type hints instead of adding type hints to
comments.

Signed-off-by: Rishabh Dave <ridave@redhat.com>