[PATCH] contrib: add a tool to detect cyclic references

[Jun Wu]

Excerpts from Jun Wu's message of 2017-05-24 12:36:20 -0700:
> [...]
> +if leakdetect.disabled:
> +    sys.exit(80, 'skipped: missing feature: cpython')

This line is wrong, will be fixed in V2.

That said, I'm more interested in how do we feel about cycles in general. My
impression is it's a best-effort to remove cycles and I plan to spend some
time on it.

> [...]