COMPAT: add >= 2.7 features
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](https://docs.python.org/2/whatsnew/2.7.html)
- 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 (#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!
