Don't install tests package

[abeyerpath]

The exclude argument to find_packages needs an iterable of package
names.

Fixes: #456

[codecov]

Codecov Report

Merging #902 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #902   +/-   ##
=======================================
  Coverage   83.41%   83.41%           
=======================================
  Files          27       27           
  Lines        3636     3636           
=======================================
  Hits         3033     3033           
  Misses        603      603           

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[simonw]

Thanks for spotting this!