...before installing the project in editable mode.

We need to avoid CI failures that occur just because some newer
dependency was installed from the requirements specified in setup.cfg.

Install the version-pinned dependencies from requirements.txt first.
Then installing the project in editable mode won't install newer
dependencies.

Change-Id: I173bf9999ace895ab536418660ea23b5269e37e2
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>