Fix rpm building for python 3

[adybbroe]

Fix so that it is possible to make an rpm package for Python 3 (only)

• Closes #xxxx
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff
• Fully documented

[coveralls]

Coverage remained the same at 91.902% when pulling 92d171c on adybbroe:python3-bdist into ef22d21 on pytroll:master.

[codecov]

Codecov Report

Merging #255 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #255   +/-   ##
=======================================
  Coverage   91.92%   91.92%           
=======================================
  Files          42       42           
  Lines        8076     8076           
=======================================
  Hits         7424     7424           
  Misses        652      652           

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[djhoese]

I assume this means that RPMs can be made for CentOS versions where Python 3 is the default. Merging now with the assumption that this doesn't break something further down the line.