Reword not-required compatibility error messages

Refer to deletion in error messages for incompatibility mutable
not-required fields in base classes, as this is simpler, and ultimately
the reason why there is an incompatibility

Author: alicederyn

mypy/semanal_typeddict.py

@@ -311,18 +311,17 @@ def verify_field_compatibility(
                 )
             else:
                 self.fail(
-                    f'Field "{field_name}" is required in base class "{source.base_name}" but not in base class "{primary_source_base_name}"',
+                    f'Field "{field_name}" is required in base class "{source.base_name}" but can be deleted in base class "{primary_source_base_name}"',
                     ctx,
                 )
         elif not source.is_required and not source.is_readonly and is_required:
             if primary_source_base_name is None:
                 self.fail(
-                    f'Field "{field_name}" is not required and not readonly in base class "{source.base_name}"',
-                    ctx,
+                    f'Field "{field_name}" can be deleted in base class "{source.base_name}"', ctx
                 )
             else:
                 self.fail(
-                    f'Field "{field_name}" is required in base class "{primary_source_base_name}" but not in base class "{source.base_name}"',
+                    f'Field "{field_name}" is required in base class "{primary_source_base_name}" but can be deleted in base class "{source.base_name}"',
                     ctx,
                 )
 

test-data/unit/check-typeddict.test

@@ -354,7 +354,7 @@ class Point1(TypedDict):
     x: int
 class Point2(TypedDict, total=False):
     x: int
-class Bad(Point1, Point2): # E: Field "x" is required in base class "Point1" but not in base class "Point2"
+class Bad(Point1, Point2): # E: Field "x" is required in base class "Point1" but can be deleted in base class "Point2"
     pass
 
 b: Bad
@@ -382,7 +382,7 @@ class Point1(TypedDict, total=False):
     x: int
 class Point2(TypedDict):
     x: int
-class Bad(Point1, Point2): # E: Field "x" is required in base class "Point2" but not in base class "Point1"
+class Bad(Point1, Point2): # E: Field "x" is required in base class "Point2" but can be deleted in base class "Point1"
     pass
 
 b: Bad
@@ -396,7 +396,7 @@ from typing import TypedDict
 class Point1(TypedDict, total=False):
     x: int
 class Point2(Point1):
-    x: int # E: Field "x" is not required and not readonly in base class "Point1"
+    x: int # E: Field "x" can be deleted in base class "Point1"
 
 p2: Point2
 reveal_type(p2) # N: Revealed type is "TypedDict('__main__.Point2', {'x': builtins.int})"