ENH: Improved `__str__` for polynomials

Add a fallback for TypeErrors that are raised when attempting
to compare arbitrary elements (e.g. strings or Python complex)
to 0 in _generate_str.

Changes the printing style of instances of the convenience classes in
the polynomial package to a more "human-readable" format.
__str__ has been modified and __format__ added to ABCPolyBase, modifying
the string representation of polynomial instances, e.g. when printed.
__repr__ and the _repr_latex method (which is used in the Jupyter
environment are unchanged.

Two print formats have been added: 'unicode' and 'ascii'. 'unicode' is
the default mode on *nix systems, and uses unicode values for numeric
subscripts and superscripts in the polynomial expression. The 'ascii'
format is the default on Windows (due to font considerations) and uses
Python-style syntax to represent powers, e.g. x**2. The default
printing style can be controlled at the package-level with the
set_default_printstyle function.

The ABCPolyBase.__str__ has also been made to respect the linewidth
printoption. Other parameters from the printoptions dictionary are not
used.

Co-Authored-By: Warren Weckesser <warren.weckesser@gmail.com>
Co-authored-by: Eric Wieser <wieser.eric@gmail.com>

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).

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.

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

assert_ is not a substitute for assert_equal