Upgrade Cython version for Python 3.7 compatibility

[sputnick1124]

Problem Description

pip install chaco fails with complaints of missing fields on PyThreadState struct when using Python 3.7. This is because Python 3.7 has rearranged its exception information.

Reproduction Steps:

pip install chaco using Python 3.7

Expected behavior:

Successful install of chaco under Python 3.7

OS, Python version:

Linux (CentOS 7)
Python 3.7.0

This has been addressed in cython upstream (issue: cython/cython#1955, PR: cython/cython#1956) and plagues a number of other projects (https://www.google.com/search?q=pythreadstate+has+no+member+named+exc_type) including enable (enthought/enable#316)

Regenerating the sources with a newer Cython fixes the issue (sputnick1124@a57b3a9)

[silverjam]

This should be fixed by #433

[sputnick1124]

Great! Thanks!