dev requirements: Bring back editable install

This partially reverts 4e889e7 and now again installs the current
project as editable in requirements/dev.txt:
* the test suite does work without this
* but running the examples is now more difficult

Fixes #2786

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

requirements/dev.txt

@@ -1,3 +1,4 @@
 -r build.txt
 -r test.txt
 -r lint.txt
+-e .