fix: enable py::implicitly_convertible<py::none, ...> for py::class_-wrapped types (#3059)

* Allow casting from None to a custom object, closes #2778

* ci.yml patch from the smart_holder branch for full CI coverage.

tests/test_methods_and_attributes.py

@@ -431,6 +431,17 @@ def test_accepts_none(msg):
     assert "incompatible function arguments" in str(excinfo.value)
 
 
+def test_casts_none(msg):
+    """#2778: implicit casting from None to object (not pointer)"""
+    a = m.NoneCastTester()
+    assert m.ok_obj_or_none(a) == -1
+    a = m.NoneCastTester(4)
+    assert m.ok_obj_or_none(a) == 4
+    a = m.NoneCastTester(None)
+    assert m.ok_obj_or_none(a) == -1
+    assert m.ok_obj_or_none(None) == -1
+
+
 def test_str_issue(msg):
     """#283: __str__ called on uninitialized instance when constructor arguments invalid"""