avro

AVRO-3148: Format Python with Black (#1233)

* AVRO-3148: Format Python with Black

Updating tox.ini to use black for lint checks going forward.

Also Ran:

```
pyupgrade --py36-plus $(git ls-files '*.py')
isort --line-length 150 --profile black $(git ls-files '*.py')
black --line-length 150 $(git ls-files '*.py')
```

* AVRO-3148: Only Typecheck/Lint on One Python

The default GitHub Actions image has trouble installing black and mypy.
Besides, lint checks only need to pass in one version of python.
So, to save time and energy, we explicitly run these in the latest
Python only.