Remove theano.sandbox.cuda containing a hidden pytest import

theano/sandbox/cuda/__init__.py

-import pytest
-
-pytest.skip(
-    "You are importing theano.sandbox.cuda. This is the old GPU back-end and "
-    "is removed from Theano. Use Theano 0.9 to use it. Even better, "
-    "transition to the new GPU back-end! See "
-    "https://github.com/Theano/Theano/wiki/Converting-to-the-new-gpu-back-end%28gpuarray%29",
-    allow_module_level=True,
-)