Upgrade flask to >=2.0 to support python 3.10 and further release

[matbgn]

Describe the feature you'd like
Upgrade flask to >=2.0 for python 3.10 compatibility. Not possible to run Kanmail with python 3.10 and more for future release.

Additional context
Flask depreciation

from collections import MutableMapping
:1: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated since Python 3.3, and in 3.10 it will stop working

if you update the flask package to >=2.0 it will import it correctly also in python 3.10

[matbgn]

I see that you're busy with projects' issues, maybe you could give me some hints to do the job for this issue as well.

What will be the corner cases to take care of when upgrading flask for Kanmail? Any other valuable advice as well?

[Fizzadar]

Thank you @matbgn, really appreciate you looking into these!

Flask wise - I can't think of anything that would cause problems, the usage of flask is pretty standard in Kanmail.

Python wise it's always pyinstaller that limits the version support, but 3.10 support is completed so this shouldn't present any issues either, so I think it should be a simple upgrade :)