Tokenize a Python source code string.

    Parameters
    ----------
    source : str
        A Python source code string

def tokenize_string(source):
    """Tokenize a Python source code string.

    Parameters
    ----------
    source : str
        A Python source code string
    """
    line_reader = StringIO(source).readline
    token_generator = tokenize.generate_tokens(line_reader)

    # Loop over all tokens till a backtick (`) is found.
    # Then, take all tokens till the next backtick to form a backtick quoted
    # string.
    for toknum, tokval, _, _, _ in token_generator:
        if tokval == '`':
            tokval = " ".join(it.takewhile(
                lambda tokval: tokval != '`',
                map(operator.itemgetter(1), token_generator)))
            toknum = _BACKTICK_QUOTED_STRING
        yield toknum, tokval