Make SecureDrop compatible with Python 3

[garrettr]

SecureDrop is currently written in and for Python 2.7. However, Python 2 will be officially EOL'ed on January 1, 2020 and we must migrate to Python 3 well before then. This work will be undertaken in stages.

Phase 1: Make SecureDrop compatible with Python 3

• Check SecureDrop's dependencies for Python 3 compatibility
• Support running the development environment and tests with Python 3
• Update developer documentation

2019-04-15 edit by @eloquence: Re-scoped Garrett's original issue per the work currently being undertaken.

[heartsucker]

Hey guys. I actually did most of this in relation an old PR that I never finished, but it wasn't that big of a change. There were just a couple of places where str and bytes had to be manually converted. Is this something that people still are interested in?

[redshiftzero]

We're still interested in this, see list of dependencies here and their Python 3 status here: #2554 (comment)

[eloquence]

For the 3/6-3/20 sprint, we're aiming to support both versions of Python in develop, with Python 2.7 as the default, but tests and development environment runnable in Python 3.5. This will require additional environment-variable based branching logic.

This work will be done in the pyyy3 branch, with @kushaldas at the helm, and the goal to merge into develop during this sprint (these changes will not be picked up for the 0.12.1 release). However, any and all dev work on this should be considered blocked until we have closed all 0.12.1 milestone issues: https://github.com/freedomofpress/securedrop/milestone/51

[redshiftzero]

#4239 made the SecureDrop web app code compatible with Python 3, closing