* 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

for more information, see https://pre-commit.ci

* fixed issues pointed out by CI

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

for more information, see https://pre-commit.ci

* fixed memory leak in default constructor

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