fix: only allow integer type_caster to call __int__ method when conversion is allowed; always call __index__ (#2698)

* Only allow integer type_caster to call __int__ or __index__ method when conversion is allowed

* Remove tests for __index__ as this seems to only be used to convert to int in 3.8+

* Take both `int` and `long` types into account for Python 2

* Add test_numpy_int_convert to assert tests currently fail, even though np.intc has an __index__ method

* Also consider __index__ as noconvert to a C++ integer

* New-style classes for Python 2.7; sigh

* Add some tests on types with custom __index__ method

* Ignore some tests in Python <3.8

* Update comment about conversion from np.float32 to C++ int

* Workaround difference between CPython and PyPy's different PyIndex_Check (unnoticed because we currently don't have PyPy >= 3.8)

* Avoid ICC segfault with py::arg()