Newly released matplotlib 3.5.0 breaks tests

[csadorf]

Matplotlib just released version 3.5.0 which breaks tests specifically this one:

aiida-core/tests/orm/data/test_data.py

Lines 90 to 106 in 01a8846

	@pytest.mark.usefixtures('clear_database_before_test')
	def test_data_exporters(data_plugin, generate_class_instance):
	"""Verify that the return value of the export methods of all `Data` sub classes have the correct type.
	It should be a tuple where the first should be a byte string and the second a dictionary.
	"""
	export_formats = data_plugin.get_export_formats()
	if not export_formats:
	return
	instance = generate_class_instance(data_plugin)
	for fileformat in export_formats:
	content, dictionary = instance._exportcontent(fileformat) # pylint: disable=protected-access
	assert isinstance(content, bytes)
	assert isinstance(dictionary, dict)