Dynamically generate --help output to list all possible version completions

[brettcannon]

As of right now, the output from --help simply lists the pattern used to specify versions:

python-launcher/src/HELP.txt

Lines 8 to 11 in a267694

	-[X] : Launch the latest Python `X` version (e.g. `-3` for the latest
	Python 3); PY_PYTHON[X] overrides what is considered the latest
	(e.g. `PY_PYTHON3=3.6` will cause `-3` to search for Python 3.6).
	-[X.Y] : Launch the specified Python version (e.g. `-3.6` for Python 3.6).

But there actually isn't anything preventing the dynamic calculation of potential executables (as used by --list), and then dynamically list all possible values, e.g.:

-3  : Launch `<path to latest Python 3>`
-3.8: Launch `<path to python3.8>`
-3.9: Launch `<path to python3.9>`

[brettcannon]

See #66 on how making --list take an argument may help with this.

[brettcannon]

Otherwise something like grep or sed might be necessary to parse out the versions.