Improve typing hints in generated code (#7)

This fixes the following runtime errors:
* Correctly set default to "None" for TiffData/UUID

and these typing errors:
* Cast default values for constrained integer attributes (both mypy and
  pywright were complaining about this)
* Drop use of conlist in ROI/Union in favor of Field (only mypy noticed this)

src/ome_autogen.py

@@ -21,14 +21,14 @@
     "XMLAnnotation": ("Optional[str]", "None", "from typing import Optional\n\n",),
     "BinData/Length": ("int", None, None),
     "ROI/Union": (
-        "conlist(Shape, min_items=1)",
+        "List[Shape] = Field(..., min_items=1)",
         None,
-        "from pydantic.types import conlist\nfrom .shape import Shape",
+        "from pydantic import Field\nfrom .shape import Shape",
     ),
     "TiffData/UUID": (
         r'Optional[dataclass(type("UUID", (), {"__annotations__": '
         r'{"file_name": str, "value": UniversallyUniqueIdentifier}}))]',
-        None,
+        "None",
         "from typing import Optional\n\nfrom "
         ".simple_types import UniversallyUniqueIdentifier",
     ),
@@ -205,6 +205,12 @@ def is_enum_type(self) -> bool:
     def is_builtin_type(self) -> bool:
         return hasattr(self.type, "python_type")
 
+    @property
+    def is_decimal(self) -> bool:
+        return self.component.type.is_derived(
+            self.component.schema.builtin_types()['decimal']
+        )
+
     @property
     def key(self):
         p = self.component.parent
@@ -247,6 +253,8 @@ def imports(self) -> Set[str]:
                 imports.add("from dataclasses import field")
         elif self.is_optional:
             imports.add("from typing import Optional")
+        if self.is_decimal:
+            imports.add("from typing import cast")
         if self.type.is_datetime():
             imports.add("from datetime import datetime")
         if not self.is_builtin_type and self.type.is_global():
@@ -324,6 +332,8 @@ def default_val_str(self) -> str:
                     default_val = f"{self.type_string}('{default_val}')"
                 elif hasattr(builtins, self.type_string):
                     default_val = repr(getattr(builtins, self.type_string)(default_val))
+                if self.is_decimal:
+                    default_val = f"cast({self.type_string}, {default_val})"
             else:
                 default_val = "None"
         return f" = {default_val}"