Add lemma `pmf_measurable` #1370

Job	Run time
rocq-core	52s
coq	55s
mathcomp-classical	2m 43s
mathcomp-analysis-single	14m 27s
mathcomp-reals	2m 11s
mathcomp-analysis	12m 29s
mathcomp-reals-stdlib	1m 39s
mathcomp-experimental-reals	1m 52s
ssprove	5m 47s
mathcomp-analysis-stdlib	2m 23s
mathcomp-infotheo	6m 48s
	52m 6s

Provide feedback