WIP: No pexpect

[javierggt]

Description

This PR changes the getenv function so it does not use pexpect.

It is possible to replace all functionality so it doesn't use pexpect, but I'm not sure we want to. The reason is that in most cases the calling code can just replace the calls to ska_shell with a line calling subprocess, and that is less work than making ska_shell pexpect-free. In case where we care about a specific behavior, I don't see the point of forcing the current ska_shell interface and behavior. For example:

• ska_shell has a logfile argument, but in this day and age we would prefer a logging.Logger or just a function.
• ska_shell updates the output in real-time, which can be convenient. I did that in runasp using subprocess, but in most cases we do not need that.
• ska_shell has an importenv argument to run_shell, but it can do that because it can make one last call to the shell instance to get the environment. In subprocess, it would require a bit of work.

If one does not care about keeping the exact arguments and behavior, then it is trivial to replace pexpect with subprocess.

Interface impacts

Testing

Unit tests

• No unit tests
• Mac
• Linux
• Windows

Independent check of unit tests by [REVIEWER NAME]

• [PLATFORM]:

Functional tests

No functional testing.