Add flags to pass on --warn-unused-ignores and --no-implicit-optional to mypy (#1421)

* Add flags to pass on --warn-unused-ignores and --no-implicit-optional to mypy
* Make implicit Optional explicit in arg types (2and3 part)
* Convert {stdlib,third_party}/2 to explicit Optional

stdlib/2and3/pdb.pyi

@@ -1,15 +1,15 @@
 # NOTE: This stub is incomplete - only contains some global functions
 
-from typing import Any, Dict
+from typing import Any, Dict, Optional
 
 def run(statement: str,
-        globals: Dict[str, Any] = None,
-        locals: Dict[str, Any] = None) -> None:
+        globals: Optional[Dict[str, Any]] = None,
+        locals: Optional[Dict[str, Any]] = None) -> None:
     ...
 
 def runeval(expression: str,
-            globals: Dict[str, Any] = None,
-            locals: Dict[str, Any] = None) -> Any:
+            globals: Optional[Dict[str, Any]] = None,
+            locals: Optional[Dict[str, Any]] = None) -> Any:
     ...
 
 def runctx(statement: str,
@@ -23,7 +23,7 @@ def runcall(*args: Any, **kwds: Any) -> Any:
 def set_trace() -> None:
     ...
 
-def post_mortem(t: Any = None) -> None:
+def post_mortem(t: Optional[Any] = None) -> None:
     ...
 
 def pm() -> None: