Improve code formatting

[jraddaoui]

• Set up black.
• Fix linting issues.
• Give meaningful names to mocked functions in tests.
• Move static vars to class attributes in tests.
• Fix docstrings.

Connects to #138.

There are some things still to do if we have the time later, reflected in #67 and #147.

[codecov]

Codecov Report

Merging #148 into master will not change coverage.
The diff coverage is 85.92%.

@@           Coverage Diff           @@
##           master     #148   +/-   ##
=======================================
  Coverage   85.38%   85.38%           
=======================================
  Files          24       24           
  Lines        1074     1074           
=======================================
  Hits          917      917           
  Misses        157      157

Impacted Files	Coverage Δ
scope/urls.py	100% <ø> (ø)	⬆️
dips/migrations/0001_initial.py	100% <ø> (ø)	⬆️
dips/helpers.py	100% <100%> (ø)	⬆️
dips/templatetags/custom_tags.py	100% <100%> (ø)	⬆️
scope/__init__.py	100% <100%> (ø)	⬆️
dips/models.py	98.64% <100%> (ø)	⬆️
scope/staticfiles.py	100% <100%> (ø)	⬆️
dips/handlers.py	100% <100%> (ø)	⬆️
.../migrations/0003_alter_digitalfile_datemodified.py	100% <100%> (ø)	⬆️
search/helpers.py	100% <100%> (ø)	⬆️
... and 13 more

[sevein]

Nice 👍