Add Python 2.7 support

[exarkun]

Fixes #121

[codecov-io]

Codecov Report

Merging #122 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
+ Coverage    93.2%   93.25%   +0.05%     
==========================================
  Files          15       15              
  Lines         618      623       +5     
  Branches       60       61       +1     
==========================================
+ Hits          576      581       +5     
  Misses         26       26              
  Partials       16       16

Impacted Files	Coverage Δ
src/towncrier/_builder.py	100% <100%> (ø)	⬆️

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