🏷️ Improve type hints for get_config_actions in (base) plugins

The Sequence is covariant (which the list isn't), which helps in
inferring in concrete classes without getting errors about mismatching
signatures between parent and subclass.

src/openforms/config/data.py

@@ -3,7 +3,9 @@
 
 from django.contrib.admin.templatetags.admin_list import _boolean_icon
 
-Action: TypeAlias = tuple[str, str]
+from openforms.typing import _StrOrPromise
+
+Action: TypeAlias = tuple[_StrOrPromise, str]  # (label, reversed URL path)
 
 
 @dataclass

src/openforms/plugins/plugin.py

@@ -1,5 +1,8 @@
+from collections.abc import Sequence
+
 from django.utils.translation import gettext_lazy as _
 
+from openforms.config.data import Action
 from openforms.config.models import GlobalConfiguration
 
 from .registry import BaseRegistry
@@ -43,7 +46,7 @@ def check_config(self):
         """
         raise NotImplementedError()
 
-    def get_config_actions(self) -> list[tuple[str, str]]:
+    def get_config_actions(self) -> Sequence[Action]:
         """
         Returns a list of tuples containing the label and URL of each action
         that is related to the configuration of this plugin. This can be to

src/openforms/prefill/contrib/demo/plugin.py

@@ -5,6 +5,7 @@
 from django.utils.crypto import get_random_string
 from django.utils.translation import gettext_lazy as _
 
+from openforms.config.data import Action
 from openforms.submissions.models import Submission
 
 from ...base import BasePlugin
@@ -44,8 +45,8 @@ def get_prefill_values(
         """
         return {attr: CALLBACKS[attr]() for attr in attributes}
 
-    def check_config(self):
+    def check_config(self) -> list[Action]:
         """
         Demo config is always valid.
         """
-        pass
+        return []