Use black if we can, mention it in the contributing guide (#328)

CONTRIBUTING.md

@@ -80,9 +80,9 @@ After a short time, a chrome or chromium instance will pop up with a note about
 controlled by software.  After a bit more time it will close and then firefox will open
 and do the same.  This is normal and part of the test suite.
 
-### Source linting
+### Source formatting
 
-Source linting happens as part of the `make test` target.
+Source is automatically formatted as part of the `make test` target, using python black.
 
 ### Buildbot
 

Makefile

@@ -61,6 +61,9 @@ start-xvfb-ci:
 
 test:
 ifndef SKIPTESTS
+ifneq (, $(shell which black))
+	black $(proj_dir)
+endif
 	sudo -u momwdb dropdb test_momwdb || echo test db is gone
 	$(proj_dir)/momw/manage.py test chroniko.tests momw.tests utilz.tests
 endif

requirements.txt

@@ -1,4 +1,5 @@
 beautifulsoup4==4.6.0
+black
 Django==2.2.1
 django-csp==3.4
 django-debug-toolbar