Fix the ctypes tests. Patch from Victor Stinner. He writes:

The problem is that ctypes c_char (and c_char_p) creates unicode string
instead of byte string. I attached a proposition (patch) to change this
behaviour (use bytes for c_char).

So in next example, it will display 'bytes' and not 'str':
  from ctypes import c_buffer, c_char
  buf = c_buffer("abcdef")
  print (type(buf[0]))

Other behaviour changes:
 - repr(c_char) adds a "b"
   eg. repr(c_char('x')) is "c_char(b'x')" instead of "c_char('x')"
 - bytes is mutable whereas str is not:
   this may break some modules based on ctypes