Fix failure of str_rfind for Unicode inputs

[mdickinson]

Fixes #471

[rahulporuri]

LGTM.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (release/5.1.x@bc89672). Click here to learn what that means.
The diff coverage is 100%.

@@               Coverage Diff                @@
##             release/5.1.x     #472   +/-   ##
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  Coverage                 ?   65.28%           
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  Files                    ?       44           
  Lines                    ?     7061           
  Branches                 ?     1412           
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  Hits                     ?     4610           
  Misses                   ?     2025           
  Partials                 ?      426

Impacted Files	Coverage Δ
traits/_py2to3.py	81.35% <100%> (ø)

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