This implements the required nodes, but not the parsing logic (that'll
be a later diff).
To support `Dict`, this diff also introduces `BaseDictElement`,
`DictElement`, `StarredDictElement`, and `BaseDict`.
It also refactors some of the logic in `BaseSet` into `_BaseSetOrDict`
so that it can be shared between `BaseSet` and `BaseDict`.
`DictComp` will come later.
This is a combination of:
- Fixing some issues upstream.
- Modifying our cleanup script to carry over noqa comments, and
copying over the new versions of some files.
- A small addition to our flake8 config.
This adds support for
```
(a for b in c if d)
```
As well as various nested versions of the expression.
This does not include nodes for other comprehension types, they'll come
in later diffs.
This ended up being pretty complicated, so the parser stuff will come in
another diff.
Hopefully this should set up up nicely for dicts, sets, and lists too.
