Use "nodefaults" in CI conda env file, remove "inducer" conda channel

.test-conda-env-py3.yml

 name: test-conda-env-py3
 channels:
-- inducer
 - conda-forge
-- defaults
+- nodefaults
 
 dependencies:
 - git
-- conda-forge::numpy
-- conda-forge::sympy
+- numpy
+- sympy
 - pocl
 - islpy
 - pyopencl