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/aestransport.py

@@ -371,7 +371,10 @@ class AesEncyptionSession:
         handshake_key_bytes: bytes = base64.b64decode(handshake_key.encode("UTF-8"))
         private_key_data = base64.b64decode(keypair.get_private_key().encode("UTF-8"))
 
-        private_key = serialization.load_der_private_key(private_key_data, None, None)
+        private_key = cast(
+            rsa.RSAPrivateKey,
+            serialization.load_der_private_key(private_key_data, None, None),
+        )
         key_and_iv = private_key.decrypt(
             handshake_key_bytes, asymmetric_padding.PKCS1v15()
         )