Use black and potentially isort in this repo

[eloquence]

Per our code formatting consensus we want to gradually implement use of black across all SecureDrop repos.

• with line length set to 100
• with an ignoreRevsFile to let us skip those revisions for purposes of attribution
• with checks enforced in CI.

securedrop-client and potentially securedrop-proxy and securedrop-sdk have also been nominated for piloting isort to test automated import sorting.