TST: use explicit ClassWithNew instead of typing.Generic

typing.Generic doesn't have a __new__ method in 3.9.

Fixes #355

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)