Fix subscriptable type usage in Python <3.9 (#15604)

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

1 files changed, 1 insertions, 2 deletions

diff --git a/synapse/storage/controllers/stats.py b/synapse/storage/controllers/stats.py

index 988e44c6af..2a03528fee 100644
--- a/synapse/storage/controllers/stats.py
+++ b/synapse/storage/controllers/stats.py
@@ -13,8 +13,7 @@
 # limitations under the License.
 
 import logging
-from collections import Counter
-from typing import TYPE_CHECKING, Collection, List, Tuple
+from typing import TYPE_CHECKING, Collection, Counter, List, Tuple
 
 from synapse.api.errors import SynapseError
 from synapse.storage.database import LoggingTransaction