Add handling for protobuf==7

`protobuf` v7 was released last week, which dropped support for some of
the APIs that we use for checking field types. This fixes those accesses
so that we can still support all the way back to v5.

Fixes #410.

Ref: https://pypi.org/project/protobuf/
Ref: https://github.com/protocolbuffers/protobuf/releases/tag/v34.0

Author: stefanvanburen

protovalidate/internal/rules.py

@@ -24,6 +24,17 @@
 from buf.validate import validate_pb2
 from protovalidate.internal.cel_field_presence import InterpretedRunner, in_has
 
+# protobuf 7+ removed FieldDescriptor.label / LABEL_REPEATED in favour of is_repeated.
+if hasattr(descriptor.FieldDescriptor, "LABEL_REPEATED"):
+
+    def _is_repeated(field: descriptor.FieldDescriptor) -> bool:
+        return field.label == descriptor.FieldDescriptor.LABEL_REPEATED  # type: ignore[attr-defined]
+
+else:
+
+    def _is_repeated(field: descriptor.FieldDescriptor) -> bool:
+        return field.is_repeated  # type: ignore[attr-defined]
+
 
 class CompilationError(Exception):
     pass
@@ -155,7 +166,7 @@ def _scalar_field_value_to_cel(val: typing.Any, field: descriptor.FieldDescripto
 
 
 def _field_value_to_cel(val: typing.Any, field: descriptor.FieldDescriptor) -> celtypes.Value:
-    if field.label == descriptor.FieldDescriptor.LABEL_REPEATED:
+    if _is_repeated(field):
         if field.message_type is not None and field.message_type.GetOptions().map_entry:
             return _map_field_value_to_cel(val, field)
         return _repeated_field_value_to_cel(val, field)
@@ -165,7 +176,7 @@ def _field_value_to_cel(val: typing.Any, field: descriptor.FieldDescriptor) -> c
 def _is_empty_field(msg: message.Message, field: descriptor.FieldDescriptor) -> bool:
     if field.has_presence:
         return not _proto_message_has_field(msg, field)
-    if field.label == descriptor.FieldDescriptor.LABEL_REPEATED:
+    if _is_repeated(field):
         return len(_proto_message_get_field(msg, field)) == 0
     return _proto_message_get_field(msg, field) == field.default_value
 
@@ -194,7 +205,7 @@ def _map_field_to_cel(msg: message.Message, field: descriptor.FieldDescriptor) -
 
 
 def field_to_cel(msg: message.Message, field: descriptor.FieldDescriptor) -> celtypes.Value:
-    if field.label == descriptor.FieldDescriptor.LABEL_REPEATED:
+    if _is_repeated(field):
         return _repeated_field_to_cel(msg, field)
     elif field.message_type is not None and not _proto_message_has_field(msg, field):
         return None
@@ -493,18 +504,18 @@ def check_field_type(field: descriptor.FieldDescriptor, expected: int, wrapper_n
 
 def _is_map(field: descriptor.FieldDescriptor):
     return (
-        field.label == descriptor.FieldDescriptor.LABEL_REPEATED
+        _is_repeated(field)
         and field.message_type is not None
         and field.message_type.GetOptions().map_entry
     )
 
 
 def _is_list(field: descriptor.FieldDescriptor):
-    return field.label == descriptor.FieldDescriptor.LABEL_REPEATED and not _is_map(field)
+    return _is_repeated(field) and not _is_map(field)
 
 
 def _zero_value(field: descriptor.FieldDescriptor):
-    if field.message_type is not None and field.label != descriptor.FieldDescriptor.LABEL_REPEATED:
+    if field.message_type is not None and not _is_repeated(field):
         return _field_value_to_cel(message_factory.GetMessageClass(field.message_type)(), field)
     else:
         return _field_value_to_cel(field.default_value, field)
@@ -1030,7 +1041,7 @@ def _new_field_rule(
         field: descriptor.FieldDescriptor,
         rules: validate_pb2.FieldRules,
     ) -> FieldRules:
-        if field.label != descriptor.FieldDescriptor.LABEL_REPEATED:
+        if not _is_repeated(field):
             return self._new_scalar_field_rule(field, rules)
         if field.message_type is not None and field.message_type.GetOptions().map_entry:
             key_rules = None
@@ -1084,7 +1095,7 @@ def _new_rules(self, desc: descriptor.Descriptor) -> list[Rules]:
                 if value_field.type != descriptor.FieldDescriptor.TYPE_MESSAGE:
                     continue
                 result.append(MapValMsgRule(self, field, key_field, value_field))
-            elif field.label == descriptor.FieldDescriptor.LABEL_REPEATED:
+            elif _is_repeated(field):
                 result.append(RepeatedMsgRule(self, field))
             else:
                 result.append(SubMsgRule(self, field))