migrate CLI argument handling back to main.py

This is a bit of churn.  We spun absolutely everything out of main.py
in a series of steps, but it isn't required that the file have zero
responsibilities.

Here mycli/cli_args.py is removed, and its responsibilities brought
back to main.py.

Some values are also moved to constants.py to avoid circular imports.

Author: rolandwalker

changelog.md

@@ -4,6 +4,7 @@ Upcoming (TBD)
 Internal
 ---------
 * Factor `main.py` into several files using mixins.
+* Move CLI argument handling back to `main.py`.
 * Update Python versions used in CI.
 * Add CI on macOS.