Fix exception cause in setup.py

[cool-RR]

I recently went over Matplotlib, Pandas and NumPy, fixing a small mistake in the way that Python 3's exception chaining is used. In this project, I found only one such mistake, and fixed it here.

The mistake is this: In some parts of the code, an exception is being caught and replaced with a more user-friendly error. In these cases the syntax raise new_error from old_error needs to be used.

Python 3's exception chaining means it shows not only the traceback of the current exception, but that of the original exception (and possibly more.) This is regardless of raise from. The usage of raise from tells Python to put a more accurate message between the tracebacks. Instead of this:

During handling of the above exception, another exception occurred:

You'll get this:

The above exception was the direct cause of the following exception:

The first is inaccurate, because it signifies a bug in the exception-handling code itself, which is a separate situation than wrapping an exception.

Let me know what you think!

[codecov]

Codecov Report

Merging #150 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #150   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         1           
  Lines          105       105           
  Branches        15        15           
=========================================
  Hits           105       105           

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[cool-RR]

Actually, I gave it another thought and I think that maybe setup.py should remain Python 2-compatible.