fill out stubs for requests.structures (#1897)

Looked at https://github.com/requests/requests/blob/master/requests/structures.py.

The "data" argument to the CaseInsensitiveDict constructor is passed as is to .update(),
so I accept the same argument types as MutableMapping.update.

LookupDict is strangely named and implemented but the point seems to be that you
setattr its keys, and then can access them with both attribute access and subscripting.
See its usage in d1fb1a29ab/requests/status_codes.py (L102).

third_party/2and3/requests/structures.pyi

@@ -1,8 +1,9 @@
-from typing import Any, Iterator, MutableMapping, Tuple, TypeVar, Generic
+from typing import Any, Dict, Iterable, Iterator, Mapping, MutableMapping, Optional, Tuple, TypeVar, Union, Generic
 
 _VT = TypeVar('_VT')
 
 class CaseInsensitiveDict(MutableMapping[str, _VT], Generic[_VT]):
+    def __init__(self, data: Optional[Union[Mapping[str, _VT], Iterable[Tuple[str, _VT]]]] = ..., **kwargs: _VT) -> None: ...
     def lower_items(self) -> Iterator[Tuple[str, _VT]]: ...
     def __setitem__(self, key: str, value: _VT) -> None: ...
     def __getitem__(self, key: str) -> _VT: ...
@@ -10,8 +11,9 @@ class CaseInsensitiveDict(MutableMapping[str, _VT], Generic[_VT]):
     def __iter__(self) -> Iterator[str]: ...
     def __len__(self) -> int: ...
 
-class LookupDict(dict):
-    name = ...  # type: Any
-    def __init__(self, name=...) -> None: ...
-    def __getitem__(self, key): ...
-    def get(self, key, default=...): ...
+class LookupDict(Dict[str, _VT]):
+    name: Any
+    def __init__(self, name: Any = ...) -> None: ...
+    def __getitem__(self, key: str) -> Optional[_VT]: ...  # type: ignore
+    def __getattr__(self, attr: str) -> _VT: ...
+    def __setattr__(self, attr: str, value: _VT) -> None: ...