nose: clean up, support Python 3.10. (#8825)

Due to changes in setuptools, we need to run 2to3 ourselves.
While we're at it, lets apply relevant patches from Debian.

https://salsa.debian.org/python-team/packages/nose/

dev-python/nose/patches/debian/sphinx-html_sidebars.diff

@@ -0,0 +1,29 @@
+From: Dmitry Shachnev <mitya57@debian.org>
+Date: Sat, 8 Feb 2020 12:47:10 +0300
+Subject: html_sidebars values should be lists, not single strings
+
+The single string values were deprecated in Sphinx 1.7 and removed
+in Sphinx 2.0.
+---
+ doc/conf.py | 8 +++++++-
+ 1 file changed, 7 insertions(+), 1 deletion(-)
+
+diff --git a/doc/conf.py b/doc/conf.py
+index ac7f0cb..358414f 100644
+--- a/doc/conf.py
++++ b/doc/conf.py
+@@ -132,7 +132,13 @@ html_static_path = ['.static']
+ 
+ # Custom sidebar templates, maps document names to template names.
+ html_sidebars = {
+-    'index': 'indexsidebar.html'
++    'index': [
++        'localtoc.html',
++        'relations.html',
++        'sourcelink.html',
++        'indexsidebar.html',
++        'searchbox.html',
++        ],
+     }
+ 
+ # Additional templates that should be rendered to pages, maps page names to