celery: move Whoosh indexing task to whoosh library where it belongs Avoid bundling everything from many different layers in one big task library. This is more feasible now when we don't need kallithea.CELERY_APP set at import time.

pytest >= 4.6.6, < 5.5
pytest-sugar >= 0.9.2, < 0.10
pytest-benchmark >= 3.2.2, < 3.3
pytest-localserver >= 0.5.0, < 0.6
mock >= 3.0.0, < 4.1
Sphinx >= 1.8.0, < 3.1
WebTest >= 2.0.6, < 2.1
isort == 5.1.2
pyflakes == 2.2.0