remove support for GDAL < 3 and add 3.6 support

[gillins]

Fixes #26

This PR removes all support for versions of GDAL older than 3.0 and adds support for GDAL 3.6.

[neteler]

Thanks a lot, I have successfully created a scratch build on Fedora:

with

• the delta-diff between 1.4.15 release and master
• this PR

Test with rawhide = F38, gdal-3.6.0:

https://koji.fedoraproject.org/koji/taskinfo?taskID=94386409

• DEBUG util.py:445: gdal-devel x86_64 3.6.0-2.fc38 ...
  • (https://kojipkgs.fedoraproject.org//work/tasks/6494/94386494/root.log)

Successfully compiled!

Test with rawhide = F37, gdal-3.5.2:

https://koji.fedoraproject.org/koji/taskinfo?taskID=94387354

• DEBUG util.py:445: gdal-devel x86_64 3.5.2-1.fc37 ...
  • (https://kojipkgs.fedoraproject.org//work/tasks/7411/94387411/root.log)

Successfully compiled!