remove string_types for compability with new mkdocs

[timvink]

Closes #9

[codecov-io]

Codecov Report

Merging #9 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #9      +/-   ##
==========================================
- Coverage      96%   95.91%   -0.09%     
==========================================
  Files           2        2              
  Lines          50       49       -1     
==========================================
- Hits           48       47       -1     
  Misses          2        2

Flag	Coverage Δ
#unittests	95.91% <ø> (-0.09%)	⬇️

Impacted Files	Coverage Δ
...kdocs_git_revision_date_localized_plugin/plugin.py	100% <ø> (ø)	⬆️

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