Fix mypy warning

Our new type formatting lookup table dict is using `.get(val, str)`
where `str` is the fallback class we'd use if no direct matches are
found, but mypy finds it confusing returning a type constructor in place
of everything else.

Manual type override helps mypy be less confused about our usecase here.

ib_async/client.py

@@ -7,7 +7,7 @@
 import struct
 import time
 from collections import deque
-from typing import Deque, List, Optional
+from typing import Deque, List, Optional, Callable, Any
 
 from eventkit import Event
 
@@ -252,7 +252,7 @@ def send(self, *fields, makeEmpty=True):
             raise ConnectionError("Not connected")
 
         # fmt: off
-        FORMAT_HANDLERS = {
+        FORMAT_HANDLERS: dict[Any, Callable[[Any], str]] = {
             # Contracts are formatted in IBKR null delimiter format
             Contract: lambda c: "\0".join([
                 str(f)