def _dnsname_to_stdlib(name):
    """
    Converts a dNSName SubjectAlternativeName field to the form used by the
    standard library on the given Python version.

    Cryptography produces a dNSName as a unicode string that was idna-decoded
    from ASCII bytes. We need to idna-encode that string to get it back, and
    then on Python 3 we also need to convert to unicode via UTF-8 (the stdlib
    uses PyUnicode_FromStringAndSize on it, which decodes via UTF-8).

    If the name cannot be idna-encoded then we return None signalling that
    the name given should be skipped.
    """
    def idna_encode(name):
        """
        Borrowed wholesale from the Python Cryptography Project. It turns out
        that we can't just safely call `idna.encode`: it can explode for
        wildcard names. This avoids that problem.
        """
        from pipenv.patched.notpip._vendor import idna

        try:
            for prefix in [u'*.', u'.']:
                if name.startswith(prefix):
                    name = name[len(prefix):]
                    return prefix.encode('ascii') + idna.encode(name)
            return idna.encode(name)
        except idna.core.IDNAError:
            return None

    name = idna_encode(name)
    if name is None:
        return None
    elif sys.version_info >= (3, 0):
        name = name.decode('utf-8')
    return name