generalization `ball_open` (#1894) #1536

Job	Run time
rocq-core	54s
coq	44s
mathcomp-analysis-single	47s
mathcomp-classical	49s
mathcomp-reals	49s
mathcomp-analysis	44s
mathcomp-reals-stdlib	45s
mathcomp-experimental-reals	43s
ssprove	8m 7s
mathcomp-analysis-stdlib	48s
mathcomp-infotheo	49s
	15m 59s

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