Fix Py_UNREACHABLE missing in 3.6

[greshilov]

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[codecov-commenter]

Codecov Report

Merging #9 (5cb25d8) into master (0e1dc58) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master       #9   +/-   ##
=======================================
  Coverage   87.50%   87.50%           
=======================================
  Files           5        5           
  Lines         128      128           
=======================================
  Hits          112      112           
  Misses         16       16           

Flag	Coverage Δ
unit	87.50% <100.00%> (ø)

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Impacted Files	Coverage Δ
pysegmenttree/_abc.py	83.33% <100.00%> (ø)

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