* delete package ptk; rename ptk2 to ptk_shell.; leave ptk2 as alias for ptk_shell.
* SHELL_TYPE "prompt_toolkit" only; remove ptk1 specific behavior.
* Doc updates: eliminate reference to prompt-toolkit < 2.0
* update requirements files ptk>=2; test shell_style="none"
* fix ptk2 stub per code review
* Add ptk2 to list of packages to install.