Remove Python 2 support, misc clean-ups

[pllim]

Fix #131 , plus other clean-ups.

[codecov]

Codecov Report

Merging #185 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #185      +/-   ##
=========================================
- Coverage   96.93%   96.9%   -0.04%     
=========================================
  Files          13      13              
  Lines        1732    1714      -18     
=========================================
- Hits         1679    1661      -18     
  Misses         53      53

Impacted Files	Coverage Δ
synphot/observation.py	99.02% <ø> (-0.01%)	⬇️
synphot/thermal.py	100% <ø> (ø)	⬆️
synphot/exceptions.py	100% <ø> (ø)	⬆️
synphot/utils.py	97.89% <ø> (-0.03%)	⬇️
synphot/models.py	89.1% <ø> (-0.09%)	⬇️
synphot/reddening.py	98.93% <ø> (-0.02%)	⬇️
synphot/compat.py	100% <ø> (ø)	⬆️
synphot/__init__.py	100% <ø> (ø)	⬆️
synphot/config.py	100% <ø> (ø)	⬆️
synphot/spectrum.py	99.45% <100%> (-0.01%)	⬇️
... and 3 more

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[pllim]