fix slice without start, cover in test

Author: acolomb

canopen/variable.py

@@ -182,7 +182,7 @@ def _get_bits(key: Union[slice, int, str, Collection[int]]) -> Union[str, Collec
             if key.stop is None:
                 raise IndexError("Bits cannot be enumerated from open-ended slice")
             else:
-                return range(key.start, key.stop, key.step or 1)
+                return range(key.start or 0, key.stop, key.step or 1)
         if isinstance(key, int):
             return [key]
         return key

test/test_variable.py

@@ -70,6 +70,7 @@ def test_bits_non_string(self):
         self.assertEqual(bits[range(1, 3)], 2)
         self.assertEqual(bits[1:3], 2)
         self.assertEqual(bits[0:3:2], 5)
+        self.assertEqual(bits[:3], 5)
         with self.assertRaises(IndexError):
             bits[1:]