Message comparison fixes

[akx]

• Add comparison operators to _NormalizedString (fixes TypeError: '<' not supported between instances of '_NormalizedString' and '_NormalizedString' #612)
• Don't crash when message.locations can't be sorted (leave them unsorted) (fixes unorderable types in write_po #606)

[codecov-io]

Codecov Report

Merging #646 into master will decrease coverage by 0.01%.
The diff coverage is 88%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage   90.55%   90.53%   -0.02%     
==========================================
  Files          24       24              
  Lines        4138     4161      +23     
==========================================
+ Hits         3747     3767      +20     
- Misses        391      394       +3

Impacted Files	Coverage Δ
babel/messages/pofile.py	95.51% <88%> (-0.69%)	⬇️

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