Python Bindings: care for python3.3+ use of ElementTree

Fedora 33 is using Python 3.9 and no longer has cElementTree.
Which was depracated in v3.3, thanks GB.

https://docs.python.org/3/library/xml.etree.elementtree.html

mythtv/bindings/python/MythTV/dataheap.py

@@ -15,7 +15,13 @@
 
 import re
 import locale
-import xml.etree.cElementTree as etree
+
+# TODO: if Python 3.3+ is in use by all distributions, use ElementTree only.
+try:
+    import xml.etree.cElementTree as etree
+except ImportError:
+    import xml.etree.ElementTree as etree
+
 from datetime import date, time
 
 _default_datetime = datetime(1900,1,1, tzinfo=datetime.UTCTZ())