STY: pep8 for npyio

Various pep8 fixes for npyio.py
Also reorganized the imports, and removed the unnecessary (I hope)
`_string_like = _is_string_like` statement.
STY: pep8 for npyio continued

Two slight style modifications in npyio, regarding line length.
BUG: Disable check for oldstyle classes in python3

Fixes gh-2561
TST: Regression test for gh-2561
TST: Raise AssertionError on failure
Merge pull request #3885 from Nodd/py3_info

BUG: Disable check for oldstyle classes in python3 (np.info)
STY: pepe8 for npyio continued 2 (typo)

fixing one typo in npyio.py
Merge pull request #3829 from jarondl/pep8-npyio

STY: pep8 for npyio
DOC: Notes on backporting.
Merge pull request #3888 from cowlicks/backporting-docs

DOC: Notes on backporting.