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(

stdlib/3/urllib/parse.pyi

@@ -25,10 +25,10 @@ class _ResultMixinBytes(_ResultMixinBase[str]):
 
 
 class _NetlocResultMixinBase(Generic[AnyStr]):
-    username: AnyStr
-    password: AnyStr
-    hostname: AnyStr
-    port: int
+    username: Optional[AnyStr]
+    password: Optional[AnyStr]
+    hostname: Optional[AnyStr]
+    port: Optional[int]
 
 class _NetlocResultMixinStr(_NetlocResultMixinBase[str], _ResultMixinStr): ...