Fix XML deprecations

Author: facelessuser

docs/change_log/release-3.4.md

@@ -111,3 +111,4 @@ The following bug fixes are included in the 3.4 release:
 * Extension entry-points are only loaded if needed (#1216).
 * Added additional checks to the `<pre><code>` handling of
   `PrettifyTreeprocessor` (#1261, #1263).
+* Fix XML deprecation warnings.

markdown/extensions/admonition.py

@@ -78,14 +78,14 @@ def parse_content(self, parent, block):
             indent = 0
             while last_child is not None:
                 if (
-                    sibling and block.startswith(' ' * self.tab_length * 2) and
-                    last_child and last_child.tag in ('ul', 'ol', 'dl')
+                    sibling is not None and block.startswith(' ' * self.tab_length * 2) and
+                    last_child is not None and last_child.tag in ('ul', 'ol', 'dl')
                 ):
 
                     # The expectation is that we'll find an `<li>` or `<dt>`.
                     # We should get its last child as well.
                     sibling = self.lastChild(last_child)
-                    last_child = self.lastChild(sibling) if sibling else None
+                    last_child = self.lastChild(sibling) if sibling is not None else None
 
                     # Context has been lost at this point, so we must adjust the
                     # text's indentation level so it will be evaluated correctly