add stub for contextvars.pyi (#1968)

Part of #1965.

stdlib/3.7/contextvars.pyi

@@ -0,0 +1,27 @@
+from typing import Any, Callable, ClassVar, Generic, Mapping, TypeVar, Union
+
+_T = TypeVar('_T')
+
+class ContextVar(Generic[_T]):
+    def __init__(self, name: str, *, default: _T = ...) -> None: ...
+    @property
+    def name(self) -> str: ...
+    def get(self, default: _T = ...) -> _T: ...
+    def set(self, value: _T) -> Token[_T]: ...
+    def reset(self, token: Token[_T]) -> None: ...
+
+class Token(Generic[_T]):
+    @property
+    def var(self) -> ContextVar[_T]: ...
+    @property
+    def old_value(self) -> Any: ...  # returns either _T or MISSING, but that's hard to express
+    MISSING: ClassVar[object]
+
+def copy_context() -> Context: ...
+
+# It doesn't make sense to make this generic, because for most Contexts each ContextVar will have
+# a different value.
+class Context(Mapping[ContextVar[Any], Any]):
+    def __init__(self) -> None: ...
+    def run(self, callable: Callable[..., _T], *args: Any, **kwargs: Any) -> _T: ...
+    def copy(self) -> Context: ...