Mark some urllib.parse return fields optional (#3332)

Per the urllib.parse documentation, username, password, hostname,
and port will be set to None if not set in the parsed URL.  The
same is true for urlparse in Python 2 according to its documentation.

stdlib/2/urlparse.pyi

@@ -1,6 +1,6 @@
 # Stubs for urlparse (Python 2)
 
-from typing import AnyStr, Dict, List, NamedTuple, Tuple, Sequence, Union, overload
+from typing import AnyStr, Dict, List, NamedTuple, Tuple, Sequence, Union, overload, Optional
 
 _String = Union[str, unicode]
 
@@ -17,13 +17,13 @@ def clear_cache() -> None: ...
 
 class ResultMixin(object):
     @property
-    def username(self) -> str: ...
+    def username(self) -> Optional[str]: ...
     @property
-    def password(self) -> str: ...
+    def password(self) -> Optional[str]: ...
     @property
-    def hostname(self) -> str: ...
+    def hostname(self) -> Optional[str]: ...
     @property
-    def port(self) -> int: ...
+    def port(self) -> Optional[int]: ...
 
 class SplitResult(
     NamedTuple(