Let pytest compilation buffer respect compilation-window-height

[swinkels]

As the title says, when the pytest compilation buffer opens, it doesn't respect a compilation-window-height that is set. As the call to display-buffer that shows the pytest compilation buffer is "buried" in python-pytest--run-as-comint, it's also not easy to override/advice that behavior.

Related, when the pytest buffer is displayed for the first time, it's not yet a compilation buffer. Because of that, Emacs also doesn't respect any window placement rules that might have been setup for compilation buffers.

I have a proof-of-concept that addresses both issues (and which I'll add in a later comment).