Fix Spotify IndexError, clean up plugin, clean up tests

[linuxdaemon]

• Fix IndexError on empty search results
• Reformat spotify.py and test_spotify.py according to Black
• Move mock_requests fixture to central conftest.py

[codecov[bot]]

Codecov Report

Merging #514 into gonzobot will increase coverage by 0.28%. The diff coverage is 100%.

@@             Coverage Diff              @@
##           gonzobot     #514      +/-   ##
============================================
+ Coverage     49.97%   50.25%   +0.28%     
============================================
  Files           228      228              
  Lines         13634    13657      +23     
============================================
+ Hits           6814     6864      +50     
+ Misses         6820     6793      -27

Impacted Files	Coverage Δ
plugins/spotify.py	84.31% <100%> (+27.31%)	:arrow_up:
tests/plugin_tests/test_spotify.py	100% <100%> (ø)	:arrow_up:

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[A-UNDERSCORE-D]

As this errors on travis everywhere, please resolve those issues

[linuxdaemon]

@A-UNDERSCORE-D All errors resolved