fix pyright error in `scipy.cluster` #104

Job	Run time
stubtest-all	51s
lint-and-typecheck	1m 14s
stubtest (scipy.cluster)	43s
stubtest (scipy.constants)	47s
stubtest (scipy.datasets)	44s
stubtest (scipy.fft)	45s
stubtest (scipy.fftpack)	50s
stubtest (scipy.integrate)	51s
stubtest (scipy.io)	48s
stubtest (scipy.linalg)	47s
stubtest (scipy.misc)	47s
stubtest (scipy.odr)	49s
stubtest (scipy.optimize)	50s
stubtest (scipy.special)	47s
stubtest (scipy.stats.contingency scipy.stats.distributions)	49s
stubtest (scipy.version)	41s
	13m 3s

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