remove `highspy` pin #915

Job	Run time
Inhouse (pypsa)	19m 51s
Inhouse (atlite)	20m 35s
Inhouse (powerplantmatching)	18m 12s
OS (ubuntu)	20m 21s
Inhouse (linopy)	18m 24s
OS (macos)	20m 8s
	1h 57m 31s

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