The documentation refers to ASCII string as Python 3-style ASCII text object. In Python 2, that's the unicode data type. The feature was backported from Python 3. I guess the documentation was directly taken from Python 3's documentation and not updated to reflect Python 2's quirky str type.

You can use the io module to get the proper text type on Python 2 and 3.

import io
with io.open('ca.crt') as f:
    ca_crt = f.read()