Add a pybind function to clear a list. (#5153)

* Add a pybing function to clear a list.

* Add required error handling.

* Add `/* py-non-const */` as suggested by @Skylion007

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Co-authored-by: Ralf W. Grosse-Kunstleve <rwgk@google.com>

tests/test_pytypes.py

@@ -65,6 +65,8 @@ def test_list(capture, doc):
     assert lins == [1, 83, 2]
     m.list_insert_size_t(lins)
     assert lins == [1, 83, 2, 57]
+    m.list_clear(lins)
+    assert lins == []
 
     with capture:
         lst = m.get_list()