fix: add docs, tests; tweak some handler method signatures

src/wutta_corepos/app.py

@@ -36,6 +36,11 @@ class WuttaCoreposAppProvider(AppProvider):
     """
 
     def get_corepos_handler(self, **kwargs):
+        """
+        Get the configured CORE-POS integration handler.
+
+        :rtype: :class:`~wutta_corepos.handler.CoreposHandler`
+        """
         if not hasattr(self, 'corepos_handler'):
             spec = self.config.get(f'{self.appname}.corepos_handler',
                                    default='wutta_corepos.handler:CoreposHandler')