ARROW-2722: [Python] Sanitize dtype number to handle edge cases

[pitrou]

No description provided.

[wesm]

+1 this is a nice cleanup. Were you able to reproduce the user's error (I assume the unit test you added does)?

[pitrou]

Yes I was (on Windows 64).

[codecov-io]

Codecov Report

Merging #2234 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #2234      +/-   ##
=========================================
+ Coverage   84.19%   84.2%   +<.01%     
=========================================
  Files         288     288              
  Lines       44073   44074       +1     
=========================================
+ Hits        37109   37112       +3     
+ Misses       6933    6931       -2     
  Partials       31      31

Impacted Files	Coverage Δ
cpp/src/arrow/python/type_traits.h	83.33% <ø> (ø)	⬆️
python/pyarrow/tests/test_convert_pandas.py	94.89% <100%> (+0.01%)	⬆️
cpp/src/arrow/python/numpy_convert.cc	88.15% <100%> (-0.45%)	⬇️
cpp/src/arrow/python/numpy_interop.h	100% <100%> (ø)	⬆️
python/pyarrow/scalar.pxi	62.39% <0%> (+0.82%)	⬆️

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