Discuss and document policy for updating BuildStream's Python dependencies

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1130)
In GitLab by [[Gitlab user @jennis]](https://gitlab.com/jennis) on Sep 9, 2019, 15:04

## Summary

By updating to a later released version of [pluginbase](https://pypi.org/project/pluginbase/) (one of our Python dependencies) I was able to fix a `DeprecationWarning` we were seeing when running the test suite.

However, when I came to update this dependency, I wasn't sure how I should go about it. So, I read our [CONTRIBUTING guidelines](https://buildstream.gitlab.io/buildstream/CONTRIBUTING.html#updating-buildstream-s-python-dependencies) which instructs the reader **how** to update dependencies, which, consequently, updates *all* Python deps (see !1587).

It seems like this didn't cause any breakages and we've agreed that we could probably do with an update. However, we should discuss and document the policy as to **when** we update our Python dependencies.

Start with the CONTRIBUTING guidelines section on updating BuildStream's Python dependencies and review the linked GitLab issue and dependency update context, including pluginbase and !1587. Clarify when dependencies should be updated and whether updates should cover all Python dependencies, then document the agreed policy in the contributing guidance.

Written by the indexing model from the issue text.