Fix overflow in reduce.

[hameerabbasi]

Fixes #207

[hameerabbasi]

@mrocklin Reaching out to you for advice, any way to fix and/or ignore the Python 2 doctests?

[mrocklin]

Hi @hameerabbasi . My recommendation would be to just not test doctests in Python 2 and be done with it

[codecov]

Codecov Report

Merging #208 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #208      +/-   ##
==========================================
+ Coverage   97.66%   97.66%   +<.01%     
==========================================
  Files          11       11              
  Lines        1501     1502       +1     
==========================================
+ Hits         1466     1467       +1     
  Misses         35       35

Impacted Files	Coverage Δ
sparse/coo/core.py	97.15% <100%> (ø)	⬆️
sparse/compatibility.py	83.33% <100%> (ø)	⬆️
sparse/sparse_array.py	96.77% <100%> (+0.1%)	⬆️
sparse/coo/common.py	97.95% <100%> (ø)	⬆️

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