restores __invert__ to arithmetic-enabled enum, fixes #1907 (#1909)

diff --git a/tests/test_enum.py b/tests/test_enum.py
index d0989ad..2f119a3 100644
--- a/tests/test_enum.py
+++ b/tests/test_enum.py
@@ -140,6 +140,7 @@
     assert int(m.Flags.Read | m.Flags.Execute) == 5
     assert int(m.Flags.Write | m.Flags.Execute) == 3
     assert int(m.Flags.Write | 1) == 3
+    assert ~m.Flags.Write == -3
 
     state = m.Flags.Read | m.Flags.Write
     assert (state & m.Flags.Read) != 0