Mark device_classes inside devicegroupmeta as private

[rytilahti]

This container shouldn't have been exposed directly to the inheriting classes.
The cli will now use the newly introduced property to access the type objects.

[codecov-commenter]

Codecov Report

Merging #1129 (fa2d1cb) into master (26795b5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1129   +/-   ##
=======================================
  Coverage   76.11%   76.11%           
=======================================
  Files          75       75           
  Lines        8852     8852           
  Branches      762      762           
=======================================
  Hits         6738     6738           
  Misses       1930     1930           
  Partials      184      184           

Impacted Files	Coverage Δ
miio/click_common.py	44.79% <100.00%> (ø)

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