Improve interface generator handling of multiple upper case letters

Instead of converting `ACTIVATE_CONNECTION` to
`a_c_t_i_v_a_t_e__c_o_n_n_e_c_t_i_o_n` convert it to
`activate_connection`.

src/sdbus/interface_generator.py

@@ -46,21 +46,28 @@ def _camel_case_to_snake_case_generator(camel: str) -> Iterator[str]:
 
     yield first_char.lower()
 
+    last_character = first_char
+
     # Yield every character
     # if upper is encountered
-    # yield _ and lower
+    # yield _ only if previous character
+    # was not already uppercase or underscore
     while True:
         try:
             c = next(i)
         except StopIteration:
             return
 
         if c.isupper():
-            yield '_'
+            if not last_character.isupper() and not last_character == "_":
+                yield '_'
+
             yield c.lower()
         else:
             yield c
 
+        last_character = c
+
 
 def camel_case_to_snake_case(camel: str) -> str:
     return ''.join(_camel_case_to_snake_case_generator(camel))

test/test_interface_generator.py

@@ -74,10 +74,23 @@
 
 class TestConverter(TestCase):
     def test_camel_to_snake(self) -> None:
-        self.assertEqual(
-            'activate_connection',
-            camel_case_to_snake_case('ActivateConnection'),
-        )
+        with self.subTest("CamelCase"):
+            self.assertEqual(
+                'activate_connection',
+                camel_case_to_snake_case('ActivateConnection'),
+            )
+
+        with self.subTest("Already snake case"):
+            self.assertEqual(
+                'activate_connection',
+                camel_case_to_snake_case('activate_connection'),
+            )
+
+        with self.subTest("Upper snake case"):
+            self.assertEqual(
+                'activate_connection',
+                camel_case_to_snake_case('ACTIVATE_CONNECTION'),
+            )
 
     def test_interface_name_to_class(self) -> None:
         self.assertEqual(