fix ConfigProtocol checking

Blocking issue resolved by this update to mypy
python/mypy#6568
python/mypy#8578

we can now check that a DataClass object conforms to our ConfigProtocol
Protocol, allowing us to pass ally mypy checks.

batconf/manager.py

@@ -4,6 +4,7 @@
 from .source import SourceList
 
 
+@typing.runtime_checkable
 class ConfigProtocol(typing.Protocol):
     __dataclass_fields__: dict
     __module__: str

batconf/sources/dataclass.py

@@ -1,24 +1,25 @@
 from dataclasses import (
-    dataclass,
     fields,
-    is_dataclass,
     _MISSING_TYPE,
 )
 
+from typing import Any
+
 from ..source import SourceInterface
+from ..manager import ConfigProtocol
 
 
 class DataclassConfig(SourceInterface):
 
-    def __init__(self, ConfigClass: dataclass):
+    def __init__(self, ConfigClass: ConfigProtocol):
         '''Extract default values from the Config dataclass
         Properties without defaults are set to None
         '''
         self._root = ConfigClass.__module__
-        self._data = dict()
+        self._data: dict = dict()
 
         for field in fields(ConfigClass):
-            if is_dataclass(field.type):
+            if isinstance(field.type, ConfigProtocol):
                 self._data[field.name] = DataclassConfig(field.type)
             elif type(field.default) is _MISSING_TYPE:
                 self._data[field.name] = None
@@ -35,7 +36,7 @@ def get(self, key: str, module: str = None):
         else:
             path = key.split('.')
 
-        conf = self._data
+        conf: Any = self._data
         for k in path:
             if not (conf := conf.get(k)):
                 return conf