Replace deprecated Element.getiterator() with Element.iter(). (GH-3864)

`xml.etree.ElementTree.Element.getiterator()` was deprecated in Python
2.7 & 3.2 and removed in the freshly released Python 3.9.

Cython/Compiler/ModuleNode.py

@@ -501,7 +501,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             except ImportError:
                 import xml.etree.ElementTree as ET
             coverage_xml = ET.parse(coverage_xml_filename).getroot()
-            for el in coverage_xml.getiterator():
+            for el in coverage_xml.iter():
                 el.tail = None  # save some memory
         else:
             coverage_xml = None