Use a non-interactive matplotlib backend for the test suite

[nathanjmcdougall]

I tried running the autotest\plot_tests.py::pst_plot_test test locally, but I got Tkinter errors.

Details

In my experience, this is quite common under pytest.

The details are a bit murky to me, but my understanding is that Tkinter is a Python wrapper around Tcl/Tk, which consists of the C libraries and a set of Tcl scripts. Internally, Python uses environment variables like TCL_LIBRARY and TK_LIBRARY to find these scripts, but it can be a fragile affair when there are virtual environments involved, pytest etc.

The default GitHub Windows runners use Python builds which tend to have this issue.
https://github.com/orgs/community/discussions/26434

By default, matplotlib uses Tkinter as an interactive backend, but you can set it to use Agg instead, which is non-interactive (and faster). Interactivity (e.g. the ability to zoom in etc.) isn't really necessary in a test environment, since usually you usually don't even look at the plots anyway.

The CI config already uses it:

pyemu/.github/workflows/ci.yml

Line 135 in 5dd3c78

MPLBACKEND: Agg # non-interactive backend for matplotlib

So this change just brings uniformity for local tests too.

[coveralls]

coverage: 77.921% (+0.05%) from 77.871%
when pulling 07efe98 on nathanjmcdougall:fix/disable-interactive-matplotlib-backend-for-tests
into 5dd3c78 on pypest:develop.