Fix pypy workflow

Fix invalid import breaking type hint and crashing tests

Author: Rud356

.github/workflows/python-tests.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, "3.10", 3.11, 3.12, 3.13, "pypy3.10"]
+        python-version: [3.7, 3.8, 3.9, "3.10", 3.11, 3.12, 3.13, "pypy-3.10-v7.3.17"]
 
     steps:
     - uses: actions/checkout@v2

config_framework/types/variable.py

@@ -7,14 +7,15 @@
 )
 
 from . import custom_exceptions
+from .abstract.loader import AbstractLoader
 from .variable_key import VariableKey
 
 if TYPE_CHECKING:
-    from .config import BaseConfig
-    from .abstract.loader import AbstractLoader
+    from .config import BaseConfig # noqa: Used for mypy
+
 
 Var = TypeVar("Var")
-Source = TypeVar("Source", bound=Union[AbstractLoader, BaseConfig])
+Source = TypeVar("Source", bound=Union[AbstractLoader, "BaseConfig"])
 CustomSerializer = Callable[["Variable", Var], Any]
 CustomDeserializer = Callable[["Variable", Any], Var]
 CustomValidator = Callable[["Variable", Var], bool]