py/objnamedtuple: Fix segfault with empty namedtuple.

The empty tuple is usually a constant object, but named tuples must be
allocated to allow modification.  Added explicit allocation to fix this.

Also added a regression test to verify creating an empty named tuple works.

Fixes issue #7870.

Signed-off-by: Lars Haulin <lars.haulin@gmail.com>

tests/basics/namedtuple1.py

@@ -85,3 +85,8 @@ print(t.foo, t.bar)
 # Not implemented so far
 #T2 = namedtuple("TupComma", "foo,bar")
 #t = T2(1, 2)
+
+# Creating an empty namedtuple should not segfault
+T5 = namedtuple("TupEmpty", [])
+t = T5()
+print(t)