Hide more import system frames in stack traces

[mhsmith]

There is already some code to try and suppress frames from our import system, but it doesn't cover all cases. This makes stack traces much harder to read, e.g. #812 (comment).

Specifically, we should remove:

• import_override
• Everything in importlib

Newer versions of Python have a public API for manipulating tracebacks, so we should be able to remove most of these. Ideally it would also cover exceptions that that don't propagate into Java, because they may still be caught and logged.

[mhsmith]

This will be improved by #1257, leaving only the following:

• A single import_override frame at the highest-level import, inserted by Cython when the function returns.
• The importlib frames between import_module and _load_unlocked, because unlike an import statement, import_module doesn't filter the stack trace. This is a bug in upstream Python:
  • Internal importlib frames visible when module imported by import_module throws exception python/cpython#73973