Pin cython to 0.29 series

Cython just released a new 3.0 major version, and it seems to break the
`autoclass` function from pyjnius. Stick with the old version until
that's ironed out.

Fixes: #164

requirements.txt

@@ -1,3 +1,3 @@
-cython
+cython~=0.29
 virtualenv
 git+https://github.com/learningequality/python-for-android@6afd747999d5252839e79f498bd237743a1ca98b#egg=python-for-android