migrate typechecking to mypy (from pyre)

[mlin]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 9544628518

Details

• 302 of 314 (96.18%) changed or added relevant lines in 22 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 95.184%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Expr.py	14	15	93.33%
WDL/Type.py	11	12	91.67%
WDL/Value.py	14	15	93.33%
WDL/_parser.py	19	20	95.0%
WDL/runtime/backend/docker_swarm.py	16	17	94.12%
WDL/runtime/download.py	9	10	90.0%
WDL/runtime/task.py	24	25	96.0%
WDL/Error.py	14	16	87.5%
WDL/_util.py	32	35	91.43%

Totals
Change from base Build 9055682944:	-0.04%
Covered Lines:	7391
Relevant Lines:	7765

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💛 - Coveralls