UserList and UserString in 2.7 (#2246)

Fill out UserList and UserString stubs to make the classes concrete. Compare:
https://github.com/python/cpython/blob/2.7/Lib/UserList.py
https://github.com/python/cpython/blob/2.7/Lib/UserString.py

stdlib/2/UserString.pyi

@@ -1,4 +1,70 @@
 import collections
+from typing import Any, Iterable, List, MutableSequence, Sequence, Optional, overload, Text, TypeVar, Tuple, Union
 
-class UserString(collections.Sequence): ...
-class MutableString(UserString, collections.MutableSequence): ...
+_UST = TypeVar("_UST", bound=UserString)
+_MST = TypeVar("_MST", bound=MutableString)
+
+class UserString(Sequence[UserString]):
+    def __init__(self, seq: object) -> None: ...
+    def __int__(self) -> int: ...
+    def __long__(self) -> long: ...
+    def __float__(self) -> float: ...
+    def __complex__(self) -> complex: ...
+    def __hash__(self) -> int: ...
+    def __len__(self) -> int: ...
+    @overload
+    def __getitem__(self: _UST, i: int) -> _UST: ...
+    @overload
+    def __getitem__(self: _UST, s: slice) -> _UST: ...
+    def __add__(self: _UST, other: Any) -> _UST: ...
+    def __radd__(self: _UST, other: Any) -> _UST: ...
+    def __mul__(self: _UST, other: int) -> _UST: ...
+    def __rmul__(self: _UST, other: int) -> _UST: ...
+    def __mod__(self: _UST, args: Any) -> _UST: ...
+    def capitalize(self: _UST) -> _UST: ...
+    def center(self: _UST, width: int, *args: Any) -> _UST: ...
+    def count(self, sub: int, start: int = ..., end: int = ...) -> int: ...
+    def decode(self: _UST, encoding: Optional[str] = ..., errors: Optional[str] = ...) -> _UST: ...
+    def encode(self: _UST, encoding: Optional[str] = ..., errors: Optional[str] = ...) -> _UST: ...
+    def endswith(self, suffix: Text, start: int = ..., end: int = ...) -> bool: ...
+    def expandtabs(self: _UST, tabsize: int = ...) -> _UST: ...
+    def find(self, sub: Text, start: int = ..., end: int = ...) -> int: ...
+    def index(self, sub: Text, start: int = ..., end: int = ...) -> int: ...
+    def isalpha(self) -> bool: ...
+    def isalnum(self) -> bool: ...
+    def isdecimal(self) -> bool: ...
+    def isdigit(self) -> bool: ...
+    def islower(self) -> bool: ...
+    def isnumeric(self) -> bool: ...
+    def isspace(self) -> bool: ...
+    def istitle(self) -> bool: ...
+    def isupper(self) -> bool: ...
+    def join(self, seq: Iterable[Text]) -> Text: ...
+    def ljust(self: _UST, width: int, *args: Any) -> _UST: ...
+    def lower(self: _UST) -> _UST: ...
+    def lstrip(self: _UST, chars: Optional[Text] = ...) -> _UST: ...
+    def partition(self, sep: Text) -> Tuple[Text, Text, Text]: ...
+    def replace(self: _UST, old: Text, new: Text, maxsplit: int = ...) -> _UST: ...
+    def rfind(self, sub: Text, start: int = ..., end: int = ...) -> int: ...
+    def rindex(self, sub: Text, start: int = ..., end: int = ...) -> int: ...
+    def rjust(self: _UST, width: int, *args: Any) -> _UST: ...
+    def rpartition(self, sep: Text) -> Tuple[Text, Text, Text]: ...
+    def rstrip(self: _UST, chars: Optional[Text] = ...) -> _UST: ...
+    def split(self, sep: Optional[Text] = ..., maxsplit: int = ...) -> List[Text]: ...
+    def rsplit(self, sep: Optional[Text] = ..., maxsplit: int = ...) -> List[Text]: ...
+    def splitlines(self, keepends: int = ...) -> List[Text]: ...
+    def startswith(self, suffix: Text, start: int = ..., end: int = ...) -> bool: ...
+    def strip(self: _UST, chars: Optional[Text] = ...) -> _UST: ...
+    def swapcase(self: _UST) -> _UST: ...
+    def title(self: _UST) -> _UST: ...
+    def translate(self: _UST, *args: Any) -> _UST: ...
+    def upper(self: _UST) -> _UST: ...
+    def zfill(self: _UST, width: int) -> _UST: ...
+
+class MutableString(UserString, MutableSequence[MutableString]):
+    def __setitem__(self, index: Union[int, slice], sub: Any) -> None: ...
+    def __delitem__(self, index: Union[int, slice]) -> None: ...
+    def immutable(self) -> UserString: ...
+    def __iadd__(self: _MST, other: Any) -> _MST: ...
+    def __imul__(self, n: int) -> _MST: ...
+    def insert(self, index: int, value: Any) -> None: ...