Regression : ValueError: Invalid co_name '_results_bag' for created function on python 2

[smarie]

Introduced in https://github.com/smarie/python-makefun/pull/80/files#diff-bcc8a023b385d782aa46679024c444b0d2e51478e3dc467e41c7209899aab9dfR30

Result impacts some build in pytest-cases: https://github.com/smarie/python-pytest-cases/actions/runs/4247221202/jobs/7385011101#step:8:168

[smarie]

The problem is that the is_identifier function proposed is too restrictive:

def is_identifier(string):
    """
    Replacement for `str.isidentifier` when it is not available (e.g. on Python 2).
    :param string:
    :return:
    """
    if len(string) == 0 or string[0].isdigit():
        return False
    return all([s.isalnum() for s in string.split("_")])

# problem with double underscores or leading underscores
is_identifier("_hello")  # False
is_identifier("hell__o")  # False