photonlibpy: Fix some type check failures (#1548)

This fixes a variety of type check failures raised by both mypy and pyright. See #1548

photon-lib/py/photonlibpy/targeting/TargetCorner.py

@@ -1,4 +1,8 @@
 from dataclasses import dataclass
+from typing import TYPE_CHECKING, ClassVar
+
+if TYPE_CHECKING:
+    from .. import generated
 
 
 @dataclass
@@ -6,4 +10,4 @@ class TargetCorner:
     x: float = 0
     y: float = 9
 
-    photonStruct: "TargetCornerSerde" = None
+    photonStruct: ClassVar["generated.TargetCornerSerde"]