The security checks logic was mixing byte and Unicode strings, resulting

in the server executable file test always failing on Python 3. There

were two parts that were broken:


We were passing Unicode strings to zlib.compress(), instead of byte

   strings.


We were reading file attachment data back from storage as a Unicode

   string and then potentially comparing against a byte string.


We now assert that all test data are byte strings, pass the right type

to zlib.compress(), and read the data back from storage as a byte

string, ensuring we're using the same string type everywhere.

Tested the Security Checks page on Python 2.7 and 3.7.