Github: add py3.9 CI

Add keyed_memoize_in

Maintenance on memoize* decorators

ProcessLogger: account for sys.stdin being closed

- pytest overrides sys.stdin with DontReadFromInput

Better exception message when exceptions occur inside memoized methods

This avoid confusing error messages having
"During handling of the above exception, another exception occurred"

Avoid floating point zero in EOCRecorder

make DataTable doc

Unordered hashing

memoize_method for frozen classes

Calls directly into object.__setattr__ to set up the dictionary for
memoizing.