Wait for serial port close when disconnecting

[puddly]

This is basically an implementation of zigpy/zigpy#1425.

We currently have a sort of race condition where consecutive opening and closings of the serial port hide the fact that we don't actually wait for the file to completely close, instead sidestepping the issue (usually) with a small delay. This PR should properly fix the problem.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 58.33333% with 10 lines in your changes missing coverage. Please review.

Project coverage is 62.23%. Comparing base (b009196) to head (ba40a46).
Report is 7 commits behind head on dev.

Files with missing lines	Patch %	Lines
universal_silabs_flasher/cpc.py	57.14%	3 Missing ⚠️
universal_silabs_flasher/gecko_bootloader.py	50.00%	3 Missing ⚠️
universal_silabs_flasher/spinel.py	57.14%	3 Missing ⚠️
universal_silabs_flasher/common.py	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #84      +/-   ##
==========================================
+ Coverage   61.78%   62.23%   +0.44%     
==========================================
  Files          14       14              
  Lines        1748     1737      -11     
==========================================
+ Hits         1080     1081       +1     
+ Misses        668      656      -12     

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