Drop marshmallow 3.0.0 RC releases compatibility

[lafrech]

apispec/src/apispec/ext/marshmallow/openapi.py

Lines 432 to 442 in 78b1480

	# field.container was renamed to field.inner in marshmallow 3.0.0rc8
	inner_field = field.inner if hasattr(field, "inner") else field.container
	ret["items"] = self.field2property(inner_field)
	elif isinstance(field, marshmallow.fields.Dict):
	if MARSHMALLOW_VERSION_INFO[0] >= 3:
	# field.value_container was renamed to field.value_field in marshmallow 3.0.0rc8
	value_field = (
	field.value_field
	if hasattr(field, "value_field")
	else field.value_container
	)

MA3 stable is out, we don't need to support older RC versions.

We could rework this in next release.

Leaving it for now to avoid unnecessary conflicts with #478.

[sloria]

Good idea; should do this on all the marshmallow libs, probably.