TST: Unpin pluggy<1.4 for CircleCI image jobs when we can

[pllim]

• Revert TST: pin pluggy in image tests to get by future incompatibility with pytest-mpl #15940
• Also see EXP: See what happens with pytest-mpl dev #15942

astropy/tox.ini

Lines 67 to 68 in 71061de

	# TODO: remove the next line when https://github.com/matplotlib/pytest-mpl/issues/216 is resolved
	image: pluggy<1.4

Blocked by

• Upgrade to new-style hook wrappers matplotlib/pytest-mpl#216

[pllim]

@ConorMacBride said pytest-mpl just needs a new release. 🤞

[ConorMacBride]

Sorry for the delay, pytest-mpl v0.17.0 has just been released! That should be enough to unpin pluggy.