Add a way to unset environment variables when spawning a terminal
I'm using the python bindings so maybe there's a side effect in the binding, and I'm also on Terminal.spawn_sync, which the docs say are deprecated but my belated move to spawn_async is another matter. When spawning a terminal, the new terminal inherits some environment variables, including the ones that I set for the terminal emulator in os.environ. I want to set these environment variables for the surrounding program, but I don't want them propagated into the new vte terminal. I found some docs that offer the envv parameter as a way to set environment variables, but I don't see a way to fully unset them and just passing "foo=" to set the variable to an empty value isn't technically correct for some of these. Requesting some way to guarantee that an environment variable will be fully unset with the envv parameter if there isn't one already.