GitHub Actions: use Agg backend for matplotlib

[calebrob6]

No description provided.

[calebrob6]

The recent series of commits on this PR were to check to see if matplotlib would behave as expected with environment variables set in the github actions yaml files.

Specifically, I added a new tests/tmp action to "tests.yaml" that ran a tmp.py script that printed the backend that matplotlib was using. Without the MPLBACKEND: Agg environment variable set, then the Ubuntu machines would use "agg", the macOS machines would use "MacOSX", and the Windows machines would use "TkAgg" (see https://github.com/microsoft/torchgeo/runs/4316379792). With the environment variable set, then all machines would use "agg".

I've reverted the temp testing code and added the environment variable to "tests.yaml".

@adamjstewart

[adamjstewart]

Can we move the MPLBACKEND stuff into a separate PR (or just merge this PR now) so that other PRs can benefit from this change without having to wait for this PR to be finished?

[calebrob6]

Yeah sure, just merge this! (note: I do not know if the env variable should go elsewhere, just figured out how it works)

[adamjstewart]

I do not know if the env variable should go elsewhere

I think this location is fine. We could only enable it for the actual pytest step and only on Windows, but I don't see any issues with using Agg on macOS too.