Right now in str.format(), we have !s, !r, and !a to allow us to call str(), repr(), and ascii() respectively on the given expression.

I'm proposing that we add a !p conversion to have pprint.pformat() be called to convert the given expression to a 'pretty' string.

Calling
```
print(f"My dict: {d!p}")
```

is a lot more concise than:

```
import pprint
print(f"My dict: {pprint.pformat(d)}")
```

We may even be able to have a static attribute stored to change the various default kwargs of pprint.pformat().