Check that the used custom interpreters are still valid.

def validate_custom_interpreters_list(self):
        """Check that the used custom interpreters are still valid."""
        custom_list = self.get_option('custom_interpreters_list')
        valid_custom_list = []
        for value in custom_list:
            if (osp.isfile(value) and programs.is_python_interpreter(value)
                    and value != get_python_executable()):
                valid_custom_list.append(value)
        self.set_option('custom_interpreters_list', valid_custom_list)