fix mypy signatures

src/base64io/__init__.py

@@ -23,7 +23,7 @@ LOGGER_NAME = 'base64io'
 
 try:  # Python 3.5.0 and 3.5.1 have incompatible typing modules
     from types import TracebackType  # noqa pylint: disable=unused-import
-    from typing import IO, Iterable, List, Optional  # noqa pylint: disable=unused-import
+    from typing import IO, Iterable, List, Type, Optional  # noqa pylint: disable=unused-import
 except ImportError:  # pragma: no cover
     # We only actually need these imports when running the mypy checks
     pass
@@ -86,7 +86,7 @@ class Base64IO(io.IOBase):
         return self
 
     def __exit__(self, exc_type, exc_value, traceback):
-        # type: (type, BaseException, TracebackType) -> None
+        # type: (Optional[Type[BaseException]], Optional[BaseException], Optional[TracebackType]) -> bool
         """Properly close self on exit."""
         self.close()