Update mistune to the 2.0 release version (#2965)

I was hoping to do this after the release came out, and it finally happened.

Author: Roman Donchenko

ci/lib/omzdocs.py

@@ -22,11 +22,7 @@ def _get_all_ast_nodes(ast_nodes):
     for node in ast_nodes:
         yield node
         if 'children' in node:
-            # workaround for https://github.com/lepture/mistune/issues/269
-            if isinstance(node['children'], str):
-                yield {'type': 'text', 'text': node['children']}
-            else:
-                yield from _get_all_ast_nodes(node['children'])
+            yield from _get_all_ast_nodes(node['children'])
 
 def _get_text_from_ast(ast_nodes):
     def get_text_from_node(node):

ci/requirements-check-basics.txt

@@ -19,7 +19,7 @@ importlib-metadata==4.8.1
     #   stevedore
 mccabe==0.6.1
     # via flake8
-mistune==2.0.0a6
+mistune==2.0.0
     # via -r ci/requirements-documentation.in
 pathspec==0.9.0
     # via yamllint

ci/requirements-documentation.in

@@ -1,2 +1,2 @@
-mistune==2.0.0a6
+mistune~=2.0
 pyyaml