multiprocessing.managers: fix TypeVar usage (#7938)

#7928

`dict()` and `list()` just return empty dictionaries and lists (respectively) if no arguments are supplied:

```python
>>> from multiprocessing.managers import SyncManager
>>> with SyncManager() as s:
...     print(s.dict())
...
{}
```

stdlib/builtins.pyi

@@ -871,11 +871,16 @@ class list(MutableSequence[_T], Generic[_T]):
     def extend(self, __iterable: Iterable[_T]) -> None: ...
     def pop(self, __index: SupportsIndex = ...) -> _T: ...
     # Signature of `list.index` should be kept in line with `collections.UserList.index()`
+    # and multiprocessing.managers.ListProxy.index()
     def index(self, __value: _T, __start: SupportsIndex = ..., __stop: SupportsIndex = ...) -> int: ...
     def count(self, __value: _T) -> int: ...
     def insert(self, __index: SupportsIndex, __object: _T) -> None: ...
     def remove(self, __value: _T) -> None: ...
     # Signature of `list.sort` should be kept inline with `collections.UserList.sort()`
+    # and multiprocessing.managers.ListProxy.sort()
+    #
+    # Use list[SupportsRichComparisonT] for the first overload rather than [SupportsRichComparison]
+    # to work around invariance
     @overload
     def sort(self: list[SupportsRichComparisonT], *, key: None = ..., reverse: bool = ...) -> None: ...
     @overload
@@ -908,6 +913,7 @@ class list(MutableSequence[_T], Generic[_T]):
 
 class dict(MutableMapping[_KT, _VT], Generic[_KT, _VT]):
     # __init__ should be kept roughly in line with `collections.UserDict.__init__`, which has similar semantics
+    # Also multiprocessing.managers.SyncManager.dict()
     @overload
     def __init__(self) -> None: ...
     @overload