Sync typeshed (#10495)

Source commit:
python/typeshed@17bc1d8

This doesn't include some recent typeshed commits. In particular, this is from before
the split of stdlib stubs to Python 2 and 3 variants. This way we can more easily narrow
down issues it might have caused.

mypy/typeshed/stdlib/@python2/os/path.pyi

@@ -1,7 +1,6 @@
 import os
 import sys
 from _typeshed import AnyPath, BytesPath, StrPath
-from genericpath import exists as exists
 from typing import Any, AnyStr, Callable, List, Optional, Sequence, Text, Tuple, TypeVar, overload
 
 _T = TypeVar("_T")
@@ -40,6 +39,7 @@ else:
 # Also, in Python 2 mixed sequences of Text and bytes results in either Text or bytes
 # So, fall back to Any
 def commonprefix(m: Sequence[AnyPath]) -> Any: ...
+def exists(path: AnyPath) -> bool: ...
 def lexists(path: AnyPath) -> bool: ...
 
 # These return float if os.stat_float_times() == True,

mypy/typeshed/stdlib/@python2/typing.pyi

@@ -5,7 +5,6 @@ from types import CodeType, FrameType, TracebackType
 # Definitions of special type checking related constructs.  Their definitions
 # are not used, so their value does not matter.
 
-overload = object()
 Any = object()
 
 class TypeVar:
@@ -40,6 +39,7 @@ Final: _SpecialForm = ...
 _F = TypeVar("_F", bound=Callable[..., Any])
 
 def final(f: _F) -> _F: ...
+def overload(f: _F) -> _F: ...
 
 Literal: _SpecialForm = ...
 # TypedDict is a (non-subscriptable) special form.
@@ -63,11 +63,9 @@ _KT_co = TypeVar("_KT_co", covariant=True)  # Key type covariant containers.
 _VT_co = TypeVar("_VT_co", covariant=True)  # Value type covariant containers.
 _T_contra = TypeVar("_T_contra", contravariant=True)  # Ditto contravariant.
 _TC = TypeVar("_TC", bound=Type[object])
-_C = TypeVar("_C", bound=Callable[..., Any])
 
-no_type_check = object()
-
-def no_type_check_decorator(decorator: _C) -> _C: ...
+def no_type_check(f: _F) -> _F: ...
+def no_type_check_decorator(decorator: _F) -> _F: ...
 
 # Type aliases and type constructors
 
@@ -492,4 +490,4 @@ class _TypedDict(Mapping[str, object], metaclass=ABCMeta):
 def NewType(name: str, tp: Type[_T]) -> Type[_T]: ...
 
 # This itself is only available during type checking
-def type_check_only(func_or_cls: _C) -> _C: ...
+def type_check_only(func_or_cls: _F) -> _F: ...