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
DOC: Update 1.8.0 release notes.

Order highlights by significance. This is a subjective judgement.
Mention the linalg enhancements and illustrate with an example.
Update memmap.py

Clarify `memmap` docstring to better reflect the meaning of *offset*.
Merge pull request #3886 from charris/update-1.8.0-notes

DOC: Update 1.8.0 release notes.
Merge pull request #3890 from andreas-h/patch-1

Update memmap.py
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