Fixed unit tests on Windows (Appveyor)

[DonJayamanne]

Fixes #1559

This pull request:

• Has a title summarizes what is changing
• Has unit tests & code coverage is not adversely affected (within reason)
• Works on all actively maintained versions of Python (e.g. Python 2.7 & the latest Python 3 release)
• Works on Windows 10, macOS, and Linux (e.g. considered file system case-sensitivity)

[codecov]

Codecov Report

Merging #1560 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1560   +/-   ##
=======================================
  Coverage   71.43%   71.43%           
=======================================
  Files         273      273           
  Lines       12700    12700           
  Branches     2282     2282           
=======================================
  Hits         9072     9072           
  Misses       3493     3493           
  Partials      135      135

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