Use a weakref for _parent to prevent cycles.

[ssteinbach]

The _parent pointer should be a weakref. This also enforces that nothing outside of item should use the _parent pointer directly, things should go through .parent()

[reinecke]

Nice catch! I wonder if we might be able to work some profiling visibility into our CI pipeline. There are some interesting puzzle pieces available:

• https://pypi.org/project/pytest-leaks/
• https://pypi.org/project/pytest-profiling/

We'd still need to do something with the data, however.

[ssteinbach]

@reinecke if you can find a service (like codecov for coverage) that can present the data and is free for open source projects, we'd be happy to discuss integrating more metrics. As we move to C++ having static analysis be part of the PR would be pretty cool.