gdal / py version matrix additions

[dazza-codes]

Please add

• gdal 2.x with py 3.7
• gdal 3.x with py 3.6

[vincentsarago]

dear @darrenleeweber
This repo is part of a personal project, because it's freely available on Github you can edit/update/push the change you need.

I'll be happy to review any PR.

thanks

[dazza-codes]

PR #21 for example (but the branch structure of the repo is not exactly conducive to this kind of request for more branches with different branch names; makes you wonder about how to create abstractions/scripts for common elements of the builds and how to use parameterized builds)