Use enum for HeosNowPlayingMedia.type

[andrewsayre]

Description:

Updates the HeosNowPlayingMedia class so that type is of type MediaType instead of a string.

Related issue (if applicable): fixes #

Checklist:

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• Tests have been added/updated. No exclusions in .coveragerc permitted.
• README.MD updated (if necessary)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (e4cd44d) to head (a5184c7).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev      #95   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          22       22           
  Lines        1973     1978    +5     
=======================================
+ Hits         1970     1975    +5     
  Misses          3        3           

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