PYBIND11_OBJECT_CVT should use namespace for error_already_set() (#3797)

* PYBIND11_OBJECT_CVT should use namespace for error_already_set()

This change makes the macro usable outside of pybind11 namespace.

* added test for use of PYBIND11_OBJECT_CVT for classes in external to pybind11 namespaces

* Extended test_pytypes.cpp and test_pytest.py

The added test defines a dummy function that takes a custom-defined class external::float_
that uses PYBIND11_OBJECT_CVT

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fixed issues pointed out by CI

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fixed memory leak in default constructor

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_pytypes.py

@@ -634,3 +634,8 @@ def test_implementation_details():
     assert m.tuple_item_set_ssize_t() == ("emely", "edmond")
     assert m.tuple_item_get_size_t(tup) == 93
     assert m.tuple_item_set_size_t() == ("candy", "cat")
+
+
+def test_external_float_():
+    r1 = m.square_float_(2.0)
+    assert r1 == 4.0