Upgrade Cython to >=0.28.2

Backported from pandas-devgh-21688.

Author: gfyoung

ci/circle-27-compat.yaml

@@ -4,7 +4,7 @@ channels:
   - conda-forge
 dependencies:
   - bottleneck=1.0.0
-  - cython=0.24
+  - cython=0.28.2
   - jinja2=2.8
   - numexpr=2.4.4 # we test that we correctly don't use an unsupported numexpr
   - numpy=1.9.3

ci/circle-35-ascii.yaml

@@ -2,7 +2,7 @@ name: pandas
 channels:
   - defaults
 dependencies:
-  - cython
+  - cython>=0.28.2
   - nomkl
   - numpy
   - python-dateutil