support for python 3.8

[sbailey]

This PR fixes several items found while testing with python 3.8:

• a QA bug about changing dict keys while iterating over the dict keys
• use collections.abc.Callable instead of collections.Callable
  • deprecated for a long time; will break in py3.9 (but now fixed here)
  • FYI, "abc" = "Abstract Base Classes", not just a random submodule name
• Don't complain about nanomaggies as an invalid FITS unit
• in the test code, I left behind some commented hooks for how to treat a specific
  warning as an error just for those tests, useful for tracking down where the warning
  is coming from so you can know where to go to fix it.