Update project setup and port to Python 3

[jodal]

Fixes #226

[belak]

Really wish this didn't change pretty much every quote in the project - makes it harder to tell what changes are actually related to Python 3... in any sense, I'll aim to get through this today.

Thanks for submitting this!

[jodal]

The reformatting with Black is a separate commit, so you can view the changes commit by commit to get a feeling for the changes.

[belak]

Just the small comment, otherwise it looks good. Looks like the actual port was fairly painless.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@6767176). Click here to learn what that means. The diff coverage is 4.1%.

@@            Coverage Diff            @@
##             master     #227   +/-   ##
=========================================
  Coverage          ?   20.02%           
=========================================
  Files             ?        9           
  Lines             ?      754           
  Branches          ?        0           
=========================================
  Hits              ?      151           
  Misses            ?      603           
  Partials          ?        0

Impacted Files	Coverage Δ
mopidy_gmusic/translator.py	18.91% <0%> (ø)
mopidy_gmusic/commands.py	0% <0%> (ø)
mopidy_gmusic/playlists.py	17.94% <0%> (ø)
mopidy_gmusic/playback.py	54.54% <0%> (ø)
mopidy_gmusic/library.py	10.75% <2.67%> (ø)
mopidy_gmusic/scrobbler_frontend.py	45% <25%> (ø)
mopidy_gmusic/session.py	43.63% <25%> (ø)
mopidy_gmusic/backend.py	31.66% <9.09%> (ø)

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