Remove gdal

[cehbrecht]

Overview

This PR removes the gdal dependency and replaces it with:

• geotiff
• fiona

gdal was used in the validator for formats: geotiff, geojson, shapefile, gml.

This PR also skips tests for Python 3.6 and CI is run now on ubuntu-latest.

Related Issue / Discussion

Fix #398

Additional Information

See also PR #636

And noGDAL discussion.

Contribution Agreement

(as per https://github.com/geopython/pywps/blob/master/CONTRIBUTING.rst#contributions-and-licensing)

• [ ] I'd like to contribute [feature X|bugfix Y|docs|something else] to PyWPS. I confirm that my contributions to PyWPS will be compatible with the PyWPS license guidelines at the time of contribution.
• [x] I have already previously agreed to the PyWPS Contributions and Licensing Guidelines

[coveralls]

Coverage remained the same at 0.0% when pulling d8481469d552db515c1324e2e65e6825801ad432 on cehbrecht:fix-gdal-ci into ced1484299761a834cc1cc854f4d9e9c2856e8b2 on geopython:main.

[huard]

I think we can just remove the section you commented out. I don't think there's any added value to the feature processing going on in the test.