Third Public Beta - packaging cleanup, logging, new CircuitPython enhancements

• Carlos made many changes to clean up the specification for required modules used by the installer. This will make supporting and tracking Mu's dependencies MUCH easier. Thank you Carlos!
• Huge thanks to Dan Halbert of Adafruit who contributed a significant amount of refactoring to the CircuitPython mode. As a result Mu now uses the adafruit-board-toolkit module for device identification, among many other helpful changes described in Dan's pull request. Thank you Dan..!
• Carlos was on fire... he also fixed a bug in the file-copy dialog when the context menu was opened with an empty list of files.
• Carlos (again), fixed some outstanding documentation issues for supporting Raspbian Buster (and newer). These are now at https://mu.rtfd.io/.
• Carlos (again, again) tidied up various aspects of the Makefile so there is only a single source of truth for running various utilities and commands.
• Logging was another focus for this release. Now that we have a few weeks worth of crash reports we've been able to look at the parts of the application that cause most grief and add extra-logging in various locations. Tim put in a great effort to make sure the "first run" and other virtual environment based aspects of Mu now have clearer and more useful logging and throw more useful exceptions, closer to the source of the problem, for the resulting crash report. Carlos ensured the IPython kernel installation was properly logged.
• We ensured various key packages were pinned to particular versions to maximise compatibility with older versions of Python still found in schools.