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())
...
{}
```

tests/stubtest_allowlists/py3_common.txt

@@ -384,6 +384,22 @@ builtins.super.__self__
 builtins.super.__self_class__
 builtins.super.__thisclass__
 
+# These multiprocessing proxy methods have *args, **kwargs signatures at runtime,
+# But have more precise (accurate) signatures in the stub
+multiprocessing.managers.BaseListProxy.__imul__
+multiprocessing.managers.BaseListProxy.__len__
+multiprocessing.managers.BaseListProxy.__reversed__
+multiprocessing.managers.BaseListProxy.reverse
+multiprocessing.managers.BaseListProxy.sort
+multiprocessing.managers.DictProxy.__iter__
+multiprocessing.managers.DictProxy.__len__
+multiprocessing.managers.DictProxy.clear
+multiprocessing.managers.DictProxy.copy
+multiprocessing.managers.DictProxy.items
+multiprocessing.managers.DictProxy.keys
+multiprocessing.managers.DictProxy.popitem
+multiprocessing.managers.DictProxy.values
+
 # GetSetDescriptor that always raises AttributeError
 builtins.OSError.characters_written
 # Aliases for OSError