COMPAT: add >= 2.7 features

[jreback]

xref #7718
xref #11988

since we recently removed 2.6 (and 3.3) support, we are now free to use some features of later releases.
Some of these may be useful. We should investigate and create individual issues.

see here

• The syntax for set literals ({1,2,3} is a mutable set).
• Dictionary and set comprehensions ({i: i*2 for i in range(3)}).
• io library (CLN: StringIO from io instead of from StringIO #10470)
• argparse (rather then optparse)
• maybe dictviews
• memoryviews
• sysconfig
• automatic number in .format

>>> '{}:{}:{day}'.format(2009, 4, day='Sunday')
'2009:4:Sunday'

• using assertRaises as a context manager

with self.assertRaises(KeyError):
    {}['foo']

• lots of other improved testing goodies!

[kawochen]

xref #10470

[jbrockmendel]

@jreback closeable?

[jreback]

sure
though maybe nice to create a new issue of things that we might want to use but don’t have already (some of the latter on this list)