Don't use cElementTree on Python 3

It's been deprecated and will be removed in 3.9 or 3.10. 3.9.0b1 doesn't
have cElementTree. I'd like to bring it back with a deprecation warning
to drop in 3.10.

See: python/cpython#19921
Signed-off-by: Christian Heimes <christian@python.org>

src/pip/_vendor/html5lib/_utils.py

@@ -2,12 +2,15 @@
 
 from types import ModuleType
 
-from pip._vendor.six import text_type
+from pip._vendor.six import text_type, PY3
 
-try:
-    import xml.etree.cElementTree as default_etree
-except ImportError:
+if PY3:
     import xml.etree.ElementTree as default_etree
+else:
+    try:
+        import xml.etree.cElementTree as default_etree
+    except ImportError:
+        import xml.etree.ElementTree as default_etree
 
 
 __all__ = ["default_etree", "MethodDispatcher", "isSurrogatePair",