Astropy should be a required dependency

[saimn]

Ref https://github.com/astropy/pytest-astropy-header/pull/32/files#r526484941

astropy is currently an optional requirement but the plugin does nothing if astropy is not installed.
@astrofrog - Is it to avoid a cross dependency which would confuse pip ?

pytest-astropy-header/setup.cfg

Lines 28 to 31 in e753567

	[options.extras_require]
	test =
	numpy
	astropy>=4.0

[pllim]

Maybe we should just make this plugin work without astropy by copying over resolve_name. Seems convoluted to require astropy and then astropy testing requiring this also.

https://github.com/astropy/astropy/blob/40521bf9df0630b144cf9d9d7027dba1491c5aa8/astropy/utils/introspection.py#L20

[saimn]

Indeed. Actually, looking at the use of resolve_name, I think we could just use importlib.import_module instead.