Merge in changes from version 1.7.3

[hcopperm]

#153 was released but never merged. this pr currently includes the changes from #171 as well.

Original description from pr 171:

Description

We learned that a recent cert upgrade caused problems for some people who use pdagent. The changes in this pr accomplish the following

• Certs are updated
• Both the ubuntu and centos builds now run
• If people want to run builds inside Docker (which does not support systemd although it technically exists), there is now a way to specify the correct option so that the packages can be Docker-compatible
• Documentation is updated
• Update version number
• Update tests so that they run in python3 (remove throttling testing)

Original description from pr 153:

Notably this should help resolve:

• Issues related to using python and pycompile rather than their Python 3 variants (see: Ubuntu 20 Install Fails #152).
• Updated HOWTO with new gnupg paths (see: make_deb.sh invalid invokation in make ubuntu #151).
• Issues attempting to install to /lib/systemd/system when not available.

The additions/deletions from pd-queue, pd-send and their *.py iterations are mostly just a copy, with a minor change in pd-queue to leverage six's iteritems.

[joshwertheim]

Quick suggestion, can we add a user-agent to our urllib requests? Something like: https://stackoverflow.com/a/24226797

This way we can start tracking usage once this is finally deployed/released.

[hcopperm]

Quick suggestion, can we add a user-agent to our urllib requests? Something like: https://stackoverflow.com/a/24226797

This way we can start tracking usage once this is finally deployed/released.

urg despite the fact that this is a very good idea i'm gonna leave it for if we end up investing more into pdagent

[blagh]

[joshwertheim]

🚀