cq: Run pylint from tox

[postlund]

In the past, pylint was run by tox but for some reason I removed it. Not
sure why. This commits adds pylint to tox again and fixes a few issues.

This PR fails due to two regressions in pylint:

pylint-dev/pylint#4123
pylint-dev/pylint#4131

Hopefully I will be able to merge this once those have been fixed. They are tagged for pylint 2.7.2 (next minor release), so hopefully it's not that far away.

[codecov]

Codecov Report

Merging #979 (9529a17) into master (f0f7eaf) will increase coverage by 0.41%.
The diff coverage is 97.97%.

@@            Coverage Diff             @@
##           master     #979      +/-   ##
==========================================
+ Coverage   95.71%   96.12%   +0.41%     
==========================================
  Files          40       40              
  Lines        3755     3846      +91     
==========================================
+ Hits         3594     3697     +103     
+ Misses        161      149      -12     

Flag	Coverage Δ
unittests	96.12% <97.97%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pyatv/dmap/tag_definitions.py	88.88% <ø> (ø)
pyatv/support/hap_tlv8.py	100.00% <ø> (ø)
pyatv/mrp/connection.py	84.53% <71.42%> (+2.43%)	⬆️
pyatv/support/mdns.py	88.00% <92.30%> (+0.10%)	⬆️
pyatv/support/dns.py	95.41% <96.49%> (+1.59%)	⬆️
pyatv/mrp/player_state.py	99.02% <98.00%> (+0.08%)	⬆️
pyatv/dmap/__init__.py	97.73% <98.24%> (+0.60%)	⬆️
pyatv/airplay/pairing.py	100.00% <100.00%> (ø)
pyatv/airplay/srp.py	97.16% <100.00%> (+0.02%)	⬆️
pyatv/const.py	100.00% <100.00%> (ø)
... and 14 more

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