Fix ElementTree imports for Python 3.9

Importing `cElementTree` has been deprecated since Python 3.3 -
importing `ElementTree` automatically uses the fastest
implementation available - and is finally removed in Python 3.9.
Importing cElementTree directly (not as part of xml) is an even
older relic, it's for Ye Time Before ElementTree Was Added To
Python and it was instead an external module...which was before
Python 2.5.

We still need to work with Python 2.7 for now, so we use a try/
except to handle both 2.7 and 3.9 cases. Also, let's not repeat
this import 12 times in one file for some reason.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

tests/test_commit.py

@@ -3,7 +3,13 @@ import osc.oscerr
 import os
 import sys
 from common import GET, PUT, POST, DELETE, OscTestCase
-from xml.etree import cElementTree as ET
+try:
+    # Works up to Python 3.8, needed for Python < 3.3 (inc 2.7)
+    from xml.etree import cElementTree as ET
+except ImportError:
+    # will import a fast implementation from 3.3 onwards, needed
+    # for 3.9+
+    from xml.etree import ElementTree as ET
 try:
     from urllib.error import HTTPError
 except ImportError: