Fix the following `mypy` errors when running `mypy` with Python 3.7:
```
synapse/storage/controllers/stats.py:58: error: "Counter" is not subscriptable, use "typing.Counter" instead [misc]
tests/test_state.py:267: error: "dict" is not subscriptable, use "typing.Dict" instead [misc]
```
Part of https://github.com/matrix-org/synapse/issues/15603
In Python 3.9, `typing` is deprecated and the types are subscriptable (generics) by default, https://peps.python.org/pep-0585/#implementation
