Organizational refactoring - the tests for the polynomial _repr_latex
method were originally defined in test_classes.py. Based on the
descriptions of the various test files, it is a better fit in
test_printing.py.

Updated docstring to reflect that _repr_latex is used in Jupyter
environments, not IPython terminals (by default).

Cleanup from the dropping of python 2

Inheriting from object was necessary for Python 2 compatibility to use
new-style classes. In Python 3, this is unnecessary as there are no
old-style classes.

Dropping the object is more idiomatic Python.

As numpy is Python 3 only, these import statements are now unnecessary
and don't alter runtime behavior.

Choices made, and the alternatives rejected (for no particularly strong reason):

1. Show terms in ascending order, to match their internal representation
  * alternative: descending, to match convention
2. Shows 0 terms in gray
  * alternative: omit entirely
  * alternative: show normally to aid comparison
3. Write each term as `basis(ax + b)
  * alternative: write as `basis(u) ... where u = ax + b`
  * alternative: show the normalized polynomial

In future it would perhaps make sense to expose these options to the end user

That function is nose specific and has not worked since `__init__` files
were added to the tests directories.