fix the stub files for the stdlib mmap module (#5705)

In Python 3:
- The `mmap` type is `Iterable[int]`, not `Iterable[bytes]`.
- The `read_byte` method returns an `int`, and the `write_byte` method only accepts an `int` as its first and only argument.
- The `__setitem__` method accepts any `ReadableBuffer` object, not just `bytes`.

In both Python 2 and 3:
- The `__delitem__` method always raises a `TypeError`, so the proper return type is `NoReturn`.
- The `mmap` type isn't generic, so I've simplified the stubs by removing the unnecessary `_mmap` class.

stdlib/@python2/mmap.pyi

@@ -1,5 +1,5 @@
 import sys
-from typing import AnyStr, Generic, Optional, Sequence, Union
+from typing import NoReturn, Optional, Sequence, Union
 
 ACCESS_DEFAULT: int
 ACCESS_READ: int
@@ -23,7 +23,7 @@ if sys.platform != "win32":
 
     PAGESIZE: int
 
-class _mmap(Generic[AnyStr]):
+class mmap(Sequence[bytes]):
     if sys.platform == "win32":
         def __init__(
             self, fileno: int, length: int, tagname: Optional[str] = ..., access: int = ..., offset: int = ...
@@ -35,21 +35,19 @@ class _mmap(Generic[AnyStr]):
     def close(self) -> None: ...
     def flush(self, offset: int = ..., size: int = ...) -> int: ...
     def move(self, dest: int, src: int, count: int) -> None: ...
-    def read_byte(self) -> AnyStr: ...
-    def readline(self) -> AnyStr: ...
+    def read_byte(self) -> bytes: ...
+    def readline(self) -> bytes: ...
     def resize(self, newsize: int) -> None: ...
     def seek(self, pos: int, whence: int = ...) -> None: ...
     def size(self) -> int: ...
     def tell(self) -> int: ...
-    def write_byte(self, byte: AnyStr) -> None: ...
+    def write_byte(self, byte: bytes) -> None: ...
     def __len__(self) -> int: ...
-
-class mmap(_mmap[bytes], Sequence[bytes]):
     def find(self, string: bytes, start: int = ..., end: int = ...) -> int: ...
     def rfind(self, string: bytes, start: int = ..., stop: int = ...) -> int: ...
     def read(self, num: int) -> bytes: ...
     def write(self, string: bytes) -> None: ...
     def __getitem__(self, index: Union[int, slice]) -> bytes: ...
     def __getslice__(self, i: Optional[int], j: Optional[int]) -> bytes: ...
-    def __delitem__(self, index: Union[int, slice]) -> None: ...
+    def __delitem__(self, index: Union[int, slice]) -> NoReturn: ...
     def __setitem__(self, index: Union[int, slice], object: bytes) -> None: ...