gengiotypefuncs.py: Read and parse files in binary mode

Fixes this build error on macOS when inside an ssh terminal:

Traceback (most recent call last):
  File "[...]/gio/tests/gengiotypefuncs.py", line 23, in <module>
    for line in f:
  File "[...]/lib/python3.6/encodings/ascii.py", line 26, in decode
    return codecs.ascii_decode(input, self.errors)[0]
UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 2625: ordinal not in range(128)

https://bugzilla.gnome.org/show_bug.cgi?id=796328