Add --include-security-branches flag

[hugovk]

For python/release-tools#433.

Adding a flag means we can easily switch it on and off from psf-salt if needed.

[StanFromIreland]

LGTM, just a few little comments.

We'll have to be careful with the docs server, as this will extend build times by about ten hours (each branch was around 3 hours recently). That might cause following runs to hit the lockfile timeout.

[hugovk]

We'll have to be careful with the docs server, as this will extend build times by about ten hours (each branch was around 3 hours recently). That might cause following runs to hit the lockfile timeout.

Yep, let's check in after it's been running for a while, and make adjustments if needed, like move the security non-HTML to its own cron loop, and possibly less frequently like once per month.