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/no-versioned-entry-point.diff

@@ -0,0 +1,31 @@
+From: Dmitry Shachnev <mitya57@debian.org>
+Date: Sat, 8 Feb 2020 13:27:14 +0300
+Subject: Remove setuptools-3.X entry point
+
+It gets outdated with every new Python 3 default version, anyway.
+---
+ setup.py | 4 +++-
+ 1 file changed, 3 insertions(+), 1 deletion(-)
+
+diff --git a/setup.py b/setup.py
+index 765b7ae..29e29a8 100644
+--- a/setup.py
++++ b/setup.py
+@@ -26,7 +26,6 @@ try:
+         entry_points = {
+         'console_scripts': [
+             'nosetests = nose:run_exit',
+-            'nosetests%s = nose:run_exit' % py_vers_tag,
+             ],
+         'distutils.commands': [
+             ' nosetests = nose.commands:nosetests',
+@@ -34,6 +33,9 @@ try:
+         },
+         test_suite = 'nose.collector',
+         )
++    if sys.version_info.major == 2:
++        addl_args['entry_points']['console_scripts'].append(
++            'nosetests%s = nose:run_exit' % py_vers_tag)
+     addl_args.update(extra)
+ 
+     # This is required by multiprocess plugin; on Windows, if