Adding an autouse fixture to close plots after each test

[greglucas]

Automatically closing figures after each test. Implemented the suggestion from #1625 (comment)

Rationale

Will automatically close figures after each test to not reuse the same figures.

Implications

[QuLogic]

Can we have a conftest.py also in the mpl directory? We may also want to put a bit of Matplotlib testing reset into the beginning of this function too.

[greglucas]

Good suggestion, I just moved everything over to the mpl-specific directory so that it only gets called there and not in every test.

What other testing reset do you think we need? Do you want to run each test in an rccontext so that any rcparams don't get propagated as well?

[greglucas]

Just realized the actual MPL fixture is not private. I was looking at _cleanup_cm this whole time 🤦‍♂️ I just pushed a new version to bring that directly into the new conftest file.

[dopplershift]

I'm not sure how I feel about this:

1. Importing from matplotlib's conftest feels weird, I wouldn't necessarily consider that public API
2. mpl_test_settings is all of matplotlib's test settings, beyond using the figure auto-closer.

@QuLogic ?

[greglucas]

That is one of the downsides of force-pushing over things with multiple implementations. Sorry about that, Ryan. The previous iteration I had in here was simply:

@pytest.fixture(autouse=True)
def close_plots(request):
    try:
        yield
    finally:
        plt.close("all")

Actually, it looks like using MPLs version doesn't work on Python 3.5 for us either, so maybe the previous iteration is the better route anyways.

[greglucas]

I just undid the "simple" version of grabbing MPLs test fixture.

I put the yield into a context manager. Of course, on my macosx backend that broke some tests :( The fix for me locally was to force all mpl tests to use the Agg backend by default (The image testing decorator switches to TkAgg when necessary for show() calls), is this an OK approach?

[dopplershift]

This works for me. It's straightforward. The Agg switching is something I've done on MetPy as well (in a different kind of global fixture.)

[dopplershift]

I had no idea switch_backend closed all the figures, but that makes sense.

[greglucas]

Me either until I went through MPLs cleanup decorator. Everyones learning something today :)

[dopplershift]

Works for me.