Port to python3 for next major Mopidy release

[Gadgetoid]

Aims to fix #1

[jodal]

To fix the build error here, you need to:

• Go to CircleCI organization security settings, e.g. https://circleci.com/gh/organizations/pimoroni/settings#security
• Check "Yes, allow all members of my organization to (...) use third-party orbs"

You only need to do this for the first repo in your org you use with CircleCI.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6058b28). Click here to learn what that means.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##             master       #3   +/-   ##
=========================================
  Coverage          ?   88.88%           
=========================================
  Files             ?        2           
  Lines             ?       90           
  Branches          ?        0           
=========================================
  Hits              ?       80           
  Misses            ?       10           
  Partials          ?        0

Impacted Files	Coverage Δ
mopidy_raspberry_gpio/pinconfig.py	77.77% <77.77%> (ø)
mopidy_raspberry_gpio/frontend.py	96.29% <96.29%> (ø)

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