refactor: use is instead of == (#560)

PLACEHOLDER is a specific instance of an object, the test here should be "is not" instead of "!="
I am experimenting with adding ndarray support, and the equality test here causes problems.

src/betterproto/__init__.py

@@ -749,7 +749,7 @@ class Message(ABC):
                 group_current.setdefault(meta.group)
 
             value = self.__raw_get(field_name)
-            if value != PLACEHOLDER and not (meta.optional and value is None):
+            if value is not PLACEHOLDER and not (meta.optional and value is None):
                 # Found a non-sentinel value
                 all_sentinel = False