before tests with StringIO fails def get_clean_source fh for xline in

## before
# tests with StringIO fails
def get_clean_source(fh):
    for i, xline in enumerate(fh.xreadlines()):
        line_number = i + 1
        ...

## after
def get_clean_source(fh):
    for i, xline in enumerate(fh):
        line_number = i + 1
        ...