Use console_scripts for CLI

[bollwyvl]

See:

• pip installable version of validator and scorer? UniversalDependencies/tools#147 (comment)

This PR moves bin/udapy into udapi/cli.py, and reworks the naming and structure to work as a main function rather than a "pure" script.

This is then referenced in setup.cfg to use console_scripts which will handle building cross-platform scripts at install time.

[martinpopel]

Thanks a lot. I understand such modernization is needed (perhaps for other OS than Linux and Windows). However, most current users followed the recommendation to have

export PATH="$HOME/udapi-python/bin:$PATH"
export PYTHONPATH="$HOME/udapi-python/:$PYTHONPATH"

in their ~/.bashrc. Udapi does not add new requirements often, so all they have to do for update is git pull.
I understand they should use pip install -e . instead of editing $PATH and $PYTHONPATH. I plan to update the README accordingly. That said, I would like to keep Udapi backward compatible, so I would prefer to (re)create bin/udapy after merging this PR:

#!/usr/bin/env python3
"""Thin wrapper for backward compatibility.  Calls udapi.cli.main()."""
import sys
from udapi.cli import main

if __name__ == "__main__": 
    sys.exit(main())

This bin/udapy will be omitted in new PyPI releases (so that it does not clash with the auto-generated udapy script).
@bollwyvl Does this make sense?
Also, I would like to set up GitHub workflow CI tests (instead of the outdated Circle CI) before merging this PRCircle CI to test also PRs from forks.

[bollwyvl]

Sure, 27f6a4c put bin back. I manually verified it doesn't make it to the .tar.gz or .whl.