Subclass relevant classes from Enum

[kylebarron]

For #231. I'm guessing you only need the __str__ override for the MediaType class, since that's the only one that's likely to actually get printed?

[lossyrob]

Nice, thanks! Two other Enums to hit:

• Extensions
• LabelType

[lossyrob]

@kylebarron good to go on this - but could you add a CHANGELOG.md entry? Still have to add that step to a PR template...

[kylebarron]

Give me a minute to check on one thing before merging

[kylebarron]

I think this should be ready to merge, thanks

[codecov-io]

Codecov Report

Merging #261 (9f776b3) into develop (3ccb5d0) will increase coverage by 0.23%.
The diff coverage is 96.72%.

@@             Coverage Diff             @@
##           develop     #261      +/-   ##
===========================================
+ Coverage    93.73%   93.96%   +0.23%     
===========================================
  Files           30       32       +2     
  Lines         3749     3996     +247     
===========================================
+ Hits          3514     3755     +241     
- Misses         235      241       +6     

Impacted Files	Coverage Δ
pystac/layout.py	94.44% <ø> (ø)
pystac/extensions/label.py	86.36% <75.00%> (-0.23%)	⬇️
pystac/link.py	97.43% <75.00%> (-0.81%)	⬇️
pystac/media_type.py	94.11% <75.00%> (-5.89%)	⬇️
pystac/catalog.py	95.67% <95.23%> (-0.17%)	⬇️
pystac/extensions/scientific.py	97.36% <97.36%> (ø)
pystac/extensions/sat.py	98.24% <98.24%> (ø)
pystac/__init__.py	100.00% <100.00%> (ø)
pystac/collection.py	97.44% <100.00%> (+0.46%)	⬆️
pystac/extensions/__init__.py	100.00% <100.00%> (ø)
... and 8 more

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