Reorder app.py imports
Description
When testing locally on some systems, it appears that pylint fails on the order of imports for buildstream/_frontend/app.py. This MR changes the order of these imports.
This is one of the solutions that fixes #453 (closed) - others may be more appropriate.
Changes proposed in this merge request: Reorder buildstream/_frontend/app.py imports
This merge request, when approved, will close issue/bug: #453 (closed)