Convert codebase to pass mypy checks

[rytilahti]

This involves changes throughout the code base to make mypy happy,
which involved some minor refactoring in parts of the codebase.

Also included:

• Adapts the CI & the pre-commit hooks to use mypy.
• ViomiVacuum consumable container shares now more code with vacuumcontainer's impl
• Gateway discovery no more advertises pyxiaomigateway
• Yeelight initialization doesn't put up a warning about python-yeelight recommendation

Ping @syssi, @starkillerOG - it would be great if you'd give this PR a try to make sure it doesn't break anything of yours :-)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.43836% with 49 lines in your changes missing coverage. Please review.

Project coverage is 75.42%. Comparing base (dd0124c) to head (56c2a1e).
Report is 385 commits behind head on master.

Files	Patch %	Lines
miio/cooker.py	66.66%	10 Missing ⚠️
miio/viomivacuum.py	33.33%	10 Missing ⚠️
miio/gateway/gatewaydevice.py	25.00%	5 Missing and 1 partial ⚠️
miio/miioprotocol.py	50.00%	5 Missing ⚠️
miio/airhumidifier.py	0.00%	2 Missing and 1 partial ⚠️
miio/miot_device.py	70.00%	3 Missing ⚠️
miio/chuangmi_plug.py	33.33%	1 Missing and 1 partial ⚠️
miio/fan.py	33.33%	2 Missing ⚠️
miio/pwzn_relay.py	60.00%	2 Missing ⚠️
miio/vacuumcontainers.py	66.66%	2 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
- Coverage   75.45%   75.42%   -0.04%     
==========================================
  Files          73       73              
  Lines        8357     8359       +2     
  Branches      743      744       +1     
==========================================
- Hits         6306     6305       -1     
- Misses       1868     1870       +2     
- Partials      183      184       +1     

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[starkillerOG]

Sorry for taking so long.
I just tested this PR, and it seems to work just fine with respect to the gateway.