DOC: Increase guidance and detail of np.polynomial docstring

Co-authored-by: Melissa Weber Mendonça <melissawm@gmail.com>

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>

Emphasize that the class-based interface is preferred.

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

pytesttester is used by every single subpackage, so making it depend on np.testing just creates cyclic dependencies that can lead to circular imports

Relates to #11457

Numpy can now be tested using the standard

    `python -c"import numpy; numpy.test()"`

construct.

The "bench" testing with the old bench files is no longer supported.
These days we use `runtests.py` and `asv`.