Dealing with unbounded recursion in logging handlers

[steve.dower]

@graingert

I don’t know what this issue was

Thread safe warning filters

The warnings module currently doesn’t offer a thread-safe (or async-safe) way of filtering warnings. Most seriously, the catch_warnings context manager is not thread-safe. Fixing this without breaking existing code is hard to do. There is much code that accesses, mutates and even assigns warnings.filters. So, my initial strategy to fix this was to add an additional layer of filtering, using a contextvar. See my draft PR for a prototype. The design is similar to what’s done for decimal cont…