Improve traceback printing for MultipleFailures

[Zac-HD]

When Hypothesis reports multiple failures, we print the tracebacks of each error we found and then raise a MultipleFailures exception. However, it would be nice to improve these printed tracebacks!

• If we're running under pytest, we should use pytest's traceback-printing logic - and respect the --tb argument for short, native, long, skipped (etc) formatting.
• Otherwise, if rich is installed we can use that like Let's get rich hynek/structlog#349 (and it's already an optional dependency for hypothesis[cli], to syntax-highlight ghostwritten code)
• And finally we can fall back to better-exceptions like Add support for better-exceptions hynek/structlog#330

See also python/cpython#28569 (comment): in Python 3.11 we'll want to work with ExceptionGroup, but there are some open questions about how to add our extra information to the display (minimal example, targets, error explanation, etc) that are still up for discussion in bugs.python.org/issue45607