default savefig format

[bjlittle]

In the process of penning #150, I noticed that when capturing the test image to an io.Bytes in-memory buffer:

pytest-mpl/pytest_mpl/plugin.py

Lines 419 to 421 in e387618

	imgdata = io.BytesIO()
	fig.savefig(imgdata, **savefig_kwargs)

That the format kwarg to savefig isn't used.

Would you be adverse to defaulting to format='png', particularly when writing to a buffer, as (from what I understand) matplotlib will default to the rcParams to select the file-format to save to the buffer for the image, and this could be a source confusion/issues for hashing repeatability for developers when testing.

i.e., what I'm proposing is that a format='png' is injected into the savefig_kwargs, iff not specified by the caller as a kwarg option in the marker decorator.

[bjlittle]

BTW I'm happy to take this on, if you guys are in favour of adopting this approach 👍

[dopplershift]

Seems fine to me.

[Cadair]

Yeah sounds good.