core: drop sys.pyi from mocks (fixes #934)

A change introduced in mypy 0.770 disallows overriding some system
modules, including `sys`.

We only need the override for `sys.print_exception`, and the simplest
solution is to make mypy ignore that particular call.

core/mocks/sys.pyi

@@ -1,16 +0,0 @@
-from typing import *
-
-def exit(retval: Any = ...) -> None:
-    raise SystemExit()
-
-def print_exception(exc: BaseException) -> None: ...
-
-path = ...  # type: List[str]
-argv = ...  # type: List[str]
-version = ...  # type: str
-version_info = ...  # type: Tuple[int, int, int]
-implementation = ...  # type: Tuple[str, Tuple[int, int, int]]
-platform = ...  # type: str
-byteorder = ...  # type: str
-maxsize = ...  # type: int
-modules = ...  # type: Dict[str, Any]

core/src/trezor/log.py

@@ -73,4 +73,5 @@ def exception(name: str, exc: BaseException) -> None:
         _log(name, DEBUG, "ui.Cancelled")
     else:
         _log(name, ERROR, "exception:")
-        sys.print_exception(exc)
+        # since mypy 0.770 we cannot override sys, so print_exception is unknown
+        sys.print_exception(exc)  # type: ignore