Convert dbus.String into python strings when importing config from keyring

[jakubfijalkowski]

Hello,

Current keyring configuration source doesn't really work - it fails with TypeError: string argument without an encoding whenever there is something mopidy-related in the keyring.

dbus package returns strings as dbus.String types that can't be byte-encoded without passing encoding explicitly. Moreover, the rest of the code doesn't really expect bytes there - it wants strings (otherwise it will not find the key in dictionary, since b'section' != 'section'). This PR converts the DBus attribute name/value pairs to strings explicitly and therefore fixes the TypeError: string argument without an encoding error.

[codecov]

Codecov Report

Merging #1887 into develop will not change coverage by %.
The diff coverage is 0.00%.

@@           Coverage Diff            @@
##           develop    #1887   +/-   ##
========================================
  Coverage    76.89%   76.89%           
========================================
  Files           55       55           
  Lines         4666     4666           
========================================
  Hits          3588     3588           
  Misses        1078     1078           

Impacted Files	Coverage Δ
mopidy/config/keyring.py	17.34% <0.00%> (ø)

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[jodal]

The keyring part isn't the most well-maintained part of Mopidy :-/

This looks good to me.