bpo-37504: Fix documentation build with texinfo builder (GH-14606)

In the table model used by docutils, the `cols` attribute of `tgroup`
nodes is mandatory, see [1]. It is used in texinfo builder in [2].

[1]: https://www.oasis-open.org/specs/tm9901.htm#AEN348
[2]: https://github.com/sphinx-doc/sphinx/blob/v2.1.2/sphinx/writers/texinfo.py#L1129

* Doc: Add texinfo support to the Makefile
diff --git a/Doc/tools/extensions/pyspecific.py b/Doc/tools/extensions/pyspecific.py
index 975c87d..4e2ad7f 100644
--- a/Doc/tools/extensions/pyspecific.py
+++ b/Doc/tools/extensions/pyspecific.py
@@ -540,6 +540,7 @@
         nodes.colspec(colwidth=30),
         nodes.colspec(colwidth=55),
         nodes.colspec(colwidth=15),
+        cols=3,
     )
     head = nodes.thead()
     body = nodes.tbody()