test with python 3.13

[davidism]

Tests pass on Python 3.13 beta 1. Add test env for 3.13 to tox and CI.

• Remove greenlet from test dependencies. It was a leftover from before the switch to contextvars was complete.
• Pin cffi to the current commit on main. Waiting for a new release to support 3.13, see https://github.com/python-cffi/cffi/issues/71 and https://github.com/python-cffi/cffi/pull/72.
• TracebackException deprecated the exc_type attribute, which was a strong reference to the actual class. It's replaced by exc_type_str, the string representation of the class. For our purposes, this should be fine. See https://github.com/python/cpython/pull/112333.