Make mypy succeed with imports on master.py

We get little benefit from our mypy QA checks at the moment, because we skip
imports. This patch is what's needed to make mypy succeed with imports on a
single file: master.py

It also updates mypy to the current version, and enables a QA check.

Mypy bugs I encountered:

dict.update with kwargs not supported:

python/mypy#1031

property setters and getters must be adjacent:

python/mypy#1465

mitmproxy/utils/typecheck.py

@@ -1,7 +1,7 @@
 import typing
 
 
-def check_type(name: str, value: typing.Any, typeinfo: type) -> None:
+def check_type(name: str, value: typing.Any, typeinfo: typing.Any) -> None:
     """
     This function checks if the provided value is an instance of typeinfo
     and raises a TypeError otherwise.