constructors: Fix defaulted TypeVars (#7965)

From the list in https://github.com/microsoft/pyright/issues/3501

stubs/contextvars/contextvars.pyi

@@ -1,6 +1,6 @@
 import sys
 from collections.abc import Callable, Iterator, Mapping
-from typing import Any, ClassVar, Generic, TypeVar
+from typing import Any, ClassVar, Generic, TypeVar, overload
 from typing_extensions import ParamSpec, final
 
 if sys.version_info >= (3, 9):
@@ -11,7 +11,10 @@ _P = ParamSpec("_P")
 
 @final
 class ContextVar(Generic[_T]):
-    def __init__(self, name: str, *, default: _T = ...) -> None: ...
+    @overload
+    def __init__(self, name: str) -> None: ...
+    @overload
+    def __init__(self, name: str, *, default: _T) -> None: ...
     @property
     def name(self) -> str: ...
     def get(self, default: _T = ...) -> _T: ...