avro

AVRO-3162: Use Argparse to Manage Arguments (#1270)

Improve the python cli, consisting mainly of scripts/avro and avro.tool, by implementing argparse, and a `__main__.py` module.

- argparse handles arity and type exceptions more fluently at invocation time than can easily be done with sys.argv.
- argparse handles sub-commands natively, especially in python 3.7 and up.
- `__main__.py` dispatch (along with setuptools `entry_points.console_scripts` at install time) allows the cli to be invoked with both `python -m avro` and just `avro` without having an out-of-band `scripts/avro` directory.
- Just in case someone is still using scripts/avro, I've kept that around with a deprecation warning.
- I added type hints as best I can; however, the avro script has an `eval` call that we might want to get rid of, some day.