Update metadata+lockfiles to support testing pants-plugins/

[cognifloyd]

Background

This is another part of introducing pants, as discussed in various TSC meetings.

Related PRs can be found in:

• pantsbuild milestone (which includes PRs since Initialize pantsbuild #5713)
• pantsbuild label (which also includes preparatory PRs that came before adding pantsbuild)

Overview of this PR

This is a follow-up for #5842 which configured pants to find plugins in pants-plugins/.

This PR consists of 4 updates:

• Now that I'm writing tests, I need to enable installing the pants testutils.
• Finish configuring interpreter_constraints for pants-plugins/. I didn't realize that we had to configure this in both pants.toml (which I did in Configure pants to look for plugins in pants-plugins/ directory #5842) and in pants-plugins/BUILD (which this PR adds).
• The pants-plugins code does not support python3.6. Once I started adding code under pants-plugins, the compileall Makefile target fails under python 3.6. So, I just exclude pants-plugins/ from that step in the Makefile.
• The tools' lockfiles, and the pants-plugins lockfile needed to be regenerated to account for the additional interpreter_constraints and the new testutils dep.

You can review each commit separately to see each of these items (lockfile regeneration is included in the commit that triggered the changes).