Add an html5lib patch for Python 3.9 compatibility

The patch is adapted from html5lib/html5lib-python@4f92357

Closes #6407
Closes #6237

news/6728.bugfix

@@ -0,0 +1 @@
+Make vendored html5lib compatible with Python 3.9.

src/pip/_vendor/html5lib/_trie/_base.py

@@ -1,6 +1,9 @@
 from __future__ import absolute_import, division, unicode_literals
 
-from collections import Mapping
+try:
+    from collections.abc import Mapping
+except ImportError:  # Python 2.7
+    from collections import Mapping
 
 
 class Trie(Mapping):

src/pip/_vendor/html5lib/treebuilders/dom.py

@@ -1,7 +1,10 @@
 from __future__ import absolute_import, division, unicode_literals
 
 
-from collections import MutableMapping
+try:
+    from collections.abc import MutableMapping
+except ImportError:  # Python 2.7
+    from collections import MutableMapping
 from xml.dom import minidom, Node
 import weakref
 

tasks/vendoring/patches/html5lib.patch

@@ -0,0 +1,31 @@
+diff --git a/src/pip/_vendor/html5lib/_trie/_base.py b/src/pip/_vendor/html5lib/_trie/_base.py
+index a1158bbb..6b71975f 100644
+--- a/src/pip/_vendor/html5lib/_trie/_base.py
++++ b/src/pip/_vendor/html5lib/_trie/_base.py
+@@ -1,6 +1,9 @@
+ from __future__ import absolute_import, division, unicode_literals
+
+-from collections import Mapping
++try:
++    from collections.abc import Mapping
++except ImportError:  # Python 2.7
++    from collections import Mapping
+
+
+ class Trie(Mapping):
+diff --git a/src/pip/_vendor/html5lib/treebuilders/dom.py b/src/pip/_vendor/html5lib/treebuilders/dom.py
+index dcfac220..d8b53004 100644
+--- a/src/pip/_vendor/html5lib/treebuilders/dom.py
++++ b/src/pip/_vendor/html5lib/treebuilders/dom.py
+@@ -1,7 +1,10 @@
+ from __future__ import absolute_import, division, unicode_literals
+
+
+-from collections import MutableMapping
++try:
++    from collections.abc import MutableMapping
++except ImportError:  # Python 2.7
++    from collections import MutableMapping
+ from xml.dom import minidom, Node
+ import weakref
+