travis: Start running black (via pre-commit) in CI
travis: Start running black (via pre-commit) in CI

Nothing too funky here. This is based on what black itself does [1].

[1] https://github.com/psf/black/blob/master/.travis.yml

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

rst2pdf/rst2pdf,rst2pdf/rst2pdf