Drop support for Python 2.7 and 3.5

[astrofrog]

Python 3.6 is now required 🎉

[codecov]

Codecov Report

Merging #2075 into master will increase coverage by 0.3%.
The diff coverage is 95.48%.

@@            Coverage Diff            @@
##           master    #2075     +/-   ##
=========================================
+ Coverage   89.63%   89.94%   +0.3%     
=========================================
  Files         399      395      -4     
  Lines       38694    37878    -816     
=========================================
- Hits        34684    34068    -616     
+ Misses       4010     3810    -200

Impacted Files	Coverage Δ
glue/app/qt/mdi_area.py	56.52% <ø> (-0.63%)	⬇️
...lue/core/data_exporters/tests/test_gridded_fits.py	100% <ø> (ø)	⬆️
glue/core/tests/test_edit_subset_mode.py	100% <ø> (ø)	⬆️
glue/core/data_exporters/hdf5.py	74.28% <ø> (-0.72%)	⬇️
glue/conftest.py	53.7% <ø> (-0.85%)	⬇️
glue/utils/qt/mime.py	100% <ø> (ø)	⬆️
.../viewers/common/qt/tests/test_data_slice_widget.py	100% <ø> (ø)	⬆️
glue/utils/wcs.py	86.13% <ø> (-0.14%)	⬇️
glue/plugins/tools/python_export.py	62.5% <ø> (-2.21%)	⬇️
glue/core/data_exporters/gridded_fits.py	90.69% <ø> (-0.22%)	⬇️
... and 325 more

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