Add support to Mongoid 9.0 #48

Job	Run time
Tests (2.7, csv_22.0, stable)	43s
Tests (2.7, mongoid_8.0, stable)	48s
Tests (2.7, mongoid_9.0, stable)	36s
Tests (3.0, csv_22.0, stable)	24s
Tests (3.0, mongoid_8.0, stable)	24s
Tests (3.0, mongoid_9.0, stable)	28s
Tests (3.1, csv_22.0, stable)	24s
Tests (3.1, mongoid_8.0, stable)	36s
Tests (3.1, mongoid_9.0, stable)	28s
Tests (3.2, csv_22.0, stable)	24s
Tests (3.2, mongoid_8.0, stable)	22s
Tests (3.2, mongoid_9.0, stable)	24s
Tests (3.3, csv_22.0, stable)	34s
Tests (3.3, mongoid_8.0, stable)	25s
Tests (3.3, mongoid_9.0, stable)	28s
Tests (2.7, csv_edge, experimental)	36s
Tests (3.0, csv_edge, experimental)	23s
Tests (3.1, csv_edge, experimental)	24s
Tests (3.2, csv_edge, experimental)	23s
Tests (3.3, csv_edge, experimental)	41s
Tests (head, csv_edge, experimental)	32s
Tests (2.7, mongoid_edge, experimental)	38s
Tests (3.0, mongoid_edge, experimental)	30s
Tests (3.1, mongoid_edge, experimental)	24s
Tests (3.2, mongoid_edge, experimental)	29s
Tests (3.3, mongoid_edge, experimental)	42s
Tests (head, mongoid_edge, experimental)	27s
Coverage	2s
	13m 39s

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