Remove theano.compat and other Python 2.x compatibility code

Co-authored-by: Brandon T. Willard <971601+brandonwillard@users.noreply.github.com>

Refactor tests.tensor.test_basic

- Remove unused tests and unnecessary helper functions
- Rename camel-case tests
- Use pytest.raises

- Use `isort` to sort imports
- Remove import cycle in `theano.gpuarray`
- Remove some Python 2.x code using `pyupgrade`.

If we use Python varargs, we're limited in the number of arrays/shapes that can
be processed/given.  Now, there's a `broadcast_shape_iter` that takes a
collection of arrays/shapes and doesn't face this restriction.