ensure python=3.5 is tested

tests/conda/binder-dir/binder/environment.yml

@@ -1,2 +1,3 @@
 dependencies:
+  - python=3.5
   - numpy

tests/conda/binder-dir/verify

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 import sys
 
-assert sys.version_info[:2] == (3, 6), sys.version
+assert sys.version_info[:2] == (3, 5), sys.version
 
 import numpy