FIX: Bump 3.4 to 3.7

[larsoner]

3.4 is almost at EOL and we should have a 3.7 entry.

[codecov-io]

Codecov Report

Merging #833 into master will increase coverage by 0.41%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #833      +/-   ##
==========================================
+ Coverage   90.92%   91.33%   +0.41%     
==========================================
  Files           4        4              
  Lines         727      727              
  Branches      106      106              
==========================================
+ Hits          661      664       +3     
+ Misses         48       46       -2     
+ Partials       18       17       -1

Impacted Files	Coverage Δ
codespell_lib/_codespell.py	84.98% <0%> (+0.72%)	⬆️

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