Add update_service callback for zeroconf listener

[rytilahti]

This callback does not currently do anything, just avoids a warning
about it being missing on recent python-zeroconf versions.

Fixes #1101

[codecov-commenter]

Codecov Report

Merging #1112 (0faaf53) into master (924cb07) will decrease coverage by 0.02%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #1112      +/-   ##
==========================================
- Coverage   76.07%   76.04%   -0.03%     
==========================================
  Files          75       75              
  Lines        8722     8730       +8     
  Branches      747      748       +1     
==========================================
+ Hits         6635     6639       +4     
- Misses       1905     1909       +4     
  Partials      182      182              

Impacted Files	Coverage Δ
miio/discovery.py	48.64% <40.00%> (+0.03%)	⬆️
miio/airpurifier_miot.py	87.93% <0.00%> (-1.36%)	⬇️

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