Fix Conda test failures, move to Python 3.7

This fixed recent Conda failures due to lack of a version of pyopencl
2018.2.2 by moving from Python 3.5 to Python 3.7.

.test-conda-env-py3.yml

@@ -10,7 +10,7 @@ dependencies:
 - pocl
 - islpy
 - pyopencl
-- python=3.5
+- python=3.7
 - symengine=0.3.0
 - python-symengine=0.3.0
 # things not in here: loopy boxtree pymbolic pyfmmlib