Bug: any_exception_class might override existing class

[l0b0]

Bug Description

any_exception_class should check whether the class name exists before creating it. Right now there's a tiny but non-zero chance that it overrides an existing class, which could wreak havoc on subsequent tests within the same Python runtime instance and which might be really hard to debug.

How to Reproduce

1. Run a whole bunch of tests with different seeds.

What did you expect to happen? any_exception_class should not ever override an existing class.

What actually happened? any_exception_class will sometimes override an existing class.