Allow ndarray versions < 0.18 #1117

Job	Run time
examples	1m 22s
check-msrv	43s
lint	1m 9s
careful	1s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }} numpy1	1s
python${{ matrix.python-version }}-${{ matrix.platform.python-architecture }} ${{ matrix.platform.os }}	1s
cross-build	1s
valgrind	1s
coverage	1s
	3m 20s

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