remove `highspy` pin #894

Job	Run time
Inhouse (pypsa)	12m 8s
Inhouse (atlite)	12m 28s
OS (ubuntu)	12m 2s
Inhouse (powerplantmatching)	12m 16s
OS (macos)	11m 10s
Inhouse (linopy)	12m 48s
	1h 12m 52s

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