Configure mypy to run in virtual environment and fix resulting issues (#989)

For some time I've noticed that my IDE is reporting mypy errors that the
pre-commit hook is not picking up. This is because [mypy
mirror](https://github.com/pre-commit/mirrors-mypy) runs in an isolated
pre-commit environment which does not have dependencies installed and it
enables `--ignore-missing-imports` to avoid errors.

This is [advised against by
mypy](https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-library-stubs-or-py-typed-marker)
for obvious reasons:

> We recommend avoiding --ignore-missing-imports if possible: it’s
equivalent to adding a # type: ignore to all unresolved imports in your
codebase.

This PR configures the mypy pre-commit hook to run in the virtual
environment and addresses the additional errors identified as a result.
It also introduces a minimal mypy config into the `pyproject.toml`

[mypy errors identified without the fixes in this
PR](https://github.com/user-attachments/files/15896693/mypyerrors.txt)

kasa/httpclient.py

@@ -36,7 +36,7 @@ class HttpClient:
 
     def __init__(self, config: DeviceConfig) -> None:
         self._config = config
-        self._client_session: aiohttp.ClientSession = None
+        self._client_session: aiohttp.ClientSession | None = None
         self._jar = aiohttp.CookieJar(unsafe=True, quote_cookie=False)
         self._last_url = URL(f"http://{self._config.host}/")