gnu: Add python-pdoc3.

* gnu/packages/patches/python-pdoc3-tests.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/local.mk (python-pdoc3): New variable.

gnu/packages/patches/python-pdoc3-tests.patch

@@ -0,0 +1,42 @@
+Taken from upstream: https://github.com/pdoc3/pdoc/commit/4aa70de2221a34a3003a7e5f52a9b91965f0e359.patch.
+
+From 4aa70de2221a34a3003a7e5f52a9b91965f0e359 Mon Sep 17 00:00:00 2001
+From: Spencer Baugh <sbaugh@catern.com>
+Date: Thu, 23 Sep 2021 09:00:25 -0400
+Subject: [PATCH] TST: use explicit ClassWithNew instead of typing.Generic
+
+typing.Generic doesn't have a __new__ method in 3.9.
+
+Fixes https://github.com/pdoc3/pdoc/issues/355
+---
+ pdoc/test/__init__.py | 10 +++++++---
+ 1 file changed, 7 insertions(+), 3 deletions(-)
+
+diff --git a/pdoc/test/__init__.py b/pdoc/test/__init__.py
+index e8c3d94..8b67ab7 100644
+--- a/pdoc/test/__init__.py
++++ b/pdoc/test/__init__.py
+@@ -1043,16 +1043,20 @@ class C2:
+ 
+         self.assertEqual(pdoc.Class('C2', mod, C2).params(), ['a', 'b', 'c=None', '*', 'd=1', 'e'])
+ 
+-        class G(typing.Generic[T]):
++        class ClassWithNew:
++            def __new__(self, arg):
++                pass
++
++        class G(ClassWithNew):
+             def __init__(self, a, b, c=100):
+                 pass
+ 
+         self.assertEqual(pdoc.Class('G', mod, G).params(), ['a', 'b', 'c=100'])
+ 
+-        class G2(typing.Generic[T]):
++        class G2(ClassWithNew):
+             pass
+ 
+-        self.assertEqual(pdoc.Class('G2', mod, G2).params(), ['*args', '**kwds'])
++        self.assertEqual(pdoc.Class('G2', mod, G2).params(), ['arg'])
+ 
+     def test_url(self):
+         mod = pdoc.Module(EXAMPLE_MODULE)