Fixes for Python 3.9 (#140)

Fix issue in logic for evaluating field types affecting python 3.9

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
 

src/betterproto/__init__.py

@@ -920,9 +920,9 @@ class Message(ABC):
         """
         output: Dict[str, Any] = {}
         field_types = self._type_hints()
+        defaults = self._betterproto.default_gen
         for field_name, meta in self._betterproto.meta_by_field_name.items():
-            field_type = field_types[field_name]
-            field_is_repeated = type(field_type) is type(typing.List)
+            field_is_repeated = defaults[field_name] is list
             value = getattr(self, field_name)
             cased_name = casing(field_name).rstrip("_")  # type: ignore
             if meta.proto_type == TYPE_MESSAGE:
@@ -988,7 +988,7 @@ class Message(ABC):
                         output[cased_name] = b64encode(value).decode("utf8")
                 elif meta.proto_type == TYPE_ENUM:
                     if field_is_repeated:
-                        enum_class: Type[Enum] = field_type.__args__[0]
+                        enum_class: Type[Enum] = field_types[field_name].__args__[0]
                         if isinstance(value, typing.Iterable) and not isinstance(
                             value, str
                         ):
@@ -997,7 +997,7 @@ class Message(ABC):
                             # transparently upgrade single value to repeated
                             output[cased_name] = [enum_class(value).name]
                     else:
-                        enum_class: Type[Enum] = field_type  # noqa
+                        enum_class: Type[Enum] = field_types[field_name]  # noqa
                         output[cased_name] = enum_class(value).name
                 else:
                     output[cased_name] = value