Fix "KeyError: 'No such transport: sqlite. Did you mean sqla?'" in tests.

This is caused by Celery dropping the "sqlite" transport alias from version
4.3.0, so I've pinned an upper limit.

docker-compose.yml

@@ -1,8 +1,7 @@
 # A docker-compose recipe for MediaGoblin hacking.
 #
 # Tested on Trisquel 8 and Guix System. Currently runs Python 3 and works for
-# photos and video. Audio raises an exception "NameError: name 'audiolab' is not
-# defined".
+# images, audio and video.
 #
 # To run the system:
 #

setup.py

@@ -56,7 +56,8 @@ install_requires = [
     'pytest>=2.3.1',
     'pytest-xdist',
     'werkzeug>=0.7,<1.0.0',
-    'celery>=3.0',
+    # Celery 4.3.0 drops the "sqlite" transport alias making our tests fail.
+    'celery>=3.0,<4.3.0',
     # Jinja2 3.0.0 uses f-strings (Python 3.7 and above) but `pip install` on
     # Debian 9 doesn't seem to respect Jinja2's 'python_requires=">=3.6"' line.
     'jinja2<3.0.0',