Add mypy to pre-commit #659

Job	Run time
Test aip-encrypt on Firefox	18m 57s
Test aip-encrypt on Chrome	18m 0s
Test aip-encrypt-mirror on Firefox	13m 48s
Test aip-encrypt-mirror on Chrome	12m 41s
Test black-box on Chrome	29m 15s
Test icc on Firefox	13m 28s
Test icc on Chrome	13m 39s
Test ipc preservation on Firefox	18m 27s
Test ipc preservation on Chrome	17m 39s
Test ipc access on Firefox	17m 50s
Test ipc access on Chrome	16m 56s
Test metadata-xml on Firefox	19m 14s
Test metadata-xml on Chrome	19m 9s
Test tcc on Firefox	13m 23s
Test tcc on Chrome	12m 30s
Test tpc on Firefox	13m 53s
Test tpc on Chrome	13m 59s
Test uuids-dirs on Firefox	13m 37s
Test uuids-dirs on Chrome	13m 59s
	5h 10m 24s

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