isort: add configruation

I've attempted to keep the following considerations:

Follow PEP8 recommendations. Having a widely used style is helpful for
new contributors, for tooling, and is one of the nice features of python
as an ecosystem. PEP8 recommends that imports are on a single line:
```python
import json
import os
```
not
```python
import json, os
```
There are a number of advantages to this:
 - It's easier to read, especially when there are many imports
 - adding or subtracting imports makes smaller git changes, which makes
   rebases and merges easier

PEP8 does recommend for using multiple imports when importing from (`from X
import Y, Z`), and we do a lot of this. Therefore, I have used a format
for that which attempts to strike a balance between readability and
compactness.

While I find:
```python
from mod import (
   one,
   two,
   three,
)
```
to be the most readable import syntax, there is an argument to be made
that this often takes up a *lot* of vertical space, so the format I've
configured does the following:
```python
from mod import one, two three
from other_mod import (
    lots, of, modules, and, classes, that, are, imported,
    andthisreallylongonethatcauseawrap
)
```
By dropping to a new line we maximize the use of horizontal space, and
thus vertical space. It also avoids situations like:

```python
from mesonbuild.compilers.mixins.gnu import (GnuLikeMixin,
                                             GnuMixin)
```

Finally, I've configured isort to automatically add `from __future__
import annotations` to all python files. This has found a number of
places it's missing, and allows us to catch this in a linter instead of
having to manually inspect for that.

Author: dcbaker

.isort.cfg

@@ -0,0 +1,6 @@
+[settings]
+py_version=37
+line_length=100
+use_parentheses=True
+multi_line_output=5
+add_imports=from __future__ import annotations