Python 3.7 compatibility

[elsteveogrande]

• exc_type now named curexc_type (similar for _value, _traceback)
• StringIO is now in a different location: https://docs.python.org/3.4/library/io.html

[lubosz]

You should not edit sixel_impl.c manually, but regenerate it with:

cython sixel/converter.py -o sixel/sixel_cimpl.c

As seen in the Makefile.

Also the StringIO import should be just the one from the Python 3 standard library, without the balancing act.

For a working fork of this, check out my repo: https://github.com/lubosz/PySixel