A couple of pedantic improvements

[bswck]

I drove by and filed

• Bash improvements #197 (includes a bugfix that hasn't yet had the chance to start being a headache)
• Adhere to PEP 621 #198
• Use sys.executable as an executable for running python subprocesses #199

Let me know if you want some help with getting typing right (I see a lot of mypy errors around, especially in the strict mode; just let me know if you need some additional workforce to resolving them or if it's even any priority)