Fix collections import DeprecationWarning

Fixes this warning in Python 3.7:

```
/path/to/lib/python3.7/site-packages/sievelib/commands.py:26
  /path/to/lib/python3.7/site-packages/sievelib/commands.py:26: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import Iterable
```

sievelib/commands.py

@@ -23,9 +23,13 @@
 """
 from __future__ import unicode_literals
 
-from collections import Iterable
 import sys
 
+try:
+    from collections.abc import Iterable
+except ImportError:  # python < 3.3
+    from collections import Iterable
+
 from future.utils import python_2_unicode_compatible
 
 from . import tools