Consider locking python package versions

[kerchner]

trafficstars

#306 and #304 were reminders that python package versions, when not sufficiently locked, can vary enough to cause problems. Revisit requirements.txt and consider locking more of the package versions (and upgrading with each SFM release, as appropriate).

[kerchner]

Also: Do we ensure that the versions of these match:

• globally-installed supervisor
• supervisor installed in the virtualenv

May require documentation tweak, and also pinning version in requirements.txt At the same time, test upgrading to supervisor 3.1.3.

[dchud]

See also #222.