This does a few things with the end goal of making simplifying and
making consistent tox and 'make' methods of test or check.
Things here:
* move python programs out of bin and into their own main. Use
entry_points to get scripts written for them. One gain here is
that we no longer have python programs that are not named .py.
flake8 and friends would not check those programs by default.
* install scripts in bin/ using the setup.py scripts and adjust
snapcraft.yaml and debian packaging for that.
* declare and use PYTHON in Makefile to avoid repeating 'python3'
* declare and use CHECK_DIRS in Makefile for list of dirs to check.
* no longer run 'flake8' from 'make check' by default.
* remove the old tests/ directory.
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