mpdupdate: add support for abstract sockets

[Kladki]

Description

Adds support for abstract sockets for connecting to MPD, as documented here

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.32%. Comparing base (488b13e) to head (e835e1d).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/mpdupdate.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7044      +/-   ##
==========================================
- Coverage   77.33%   77.32%   -0.02%     
==========================================
  Files         163      163              
  Lines       21857    21859       +2     
  Branches     3372     3373       +1     
==========================================
- Hits        16904    16902       -2     
- Misses       4137     4141       +4     
  Partials      816      816              

Files with missing lines	Coverage Δ
beetsplug/mpdupdate.py	30.00% <0.00%> (-0.89%)	⬇️

... and 1 file with indirect coverage changes

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[snejus]

Maybe add a test for this?

[Kladki]

I tried to add a test for this, but tbh I couldn't figure out how, sorry.