* Use binary flag when writing cert and key files
Add binary flag to mode argument when opening files for writing key and
certificate files.
On Python 3 the data buffers use for writing are bytes objects not
strings, and the write fails accordingly.
As far as I understand, it the "b" flag will not hurt things in Python 2
either.
* Update the tests for RenewableCert::save_successor
Update the tests for RenewableCert::save_successor after changing
three parameters to be called with bytes objects instead of strings.
Also, update the doc comment of the function.
