DEP: Deprecate ndarray.tostring()

The corresponding `array.array.tostring()` in the standard library has been deprecated in favor of `tobytes` since Python 3.1 (python/cpython@1ce3eb5c5b4830e69b21865e2d723e22749544e0).

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.

This code was only here for Python 2.5 compatibility, but numpy requires 2.7
at minimum

Some of the documentation for newbyteorder, copy and pasted in several
spots, had paragraphs ending in `::`, initiating a sphinx generated
Verbatim environment and resulting in "LaTeX Error: Too deeply nested".

The user_array.container class needed non-empty class documentation.
That that caused a problem is probably a numpydoc bug, but it is easy to
fix.

[skip ci]

The rules enforced are the same as those used for scipy.

Some of those problems look like potential coding errors. In those
cases a Fixme comment was made and the offending code, usually an
unused variable, was commented out.