1.4.21
move down py/impl/XYZ to py/_XYZ
backout e2bc0b67063e
rather than monkeypatching use a copy of the involved code for traceback.format_exception_only and simplify invocation places.
make short-style tracebacks more like native and allow callers (in particular pytest) to set tb-style on a per-traceback entry basis
remove unused imports