Move script_runner fixture to session scope

[HexDecimal]

Requires its dependencies to also me moved to the session scope, tmp_path was function-scoped and must be replaced with tmp_path_factory.

Needed to work well with custom fixtures, Fixes #89

ScriptRunner attributes marked as Final to document and enforce them as read-only

I've also noticed that rootdir does not seem to be used anywhere, but I didn't do anything with this knowledge. script_cwd and rootdir can probably be removed.