Make pybind11 code generator support context managers.
Make pybind11 code generator support context managers.

This is for https://source.corp.google.com/piper///depot/google3/perftools/tracing/public/python/request_trace_span.clif;l=25.

PiperOrigin-RevId: 437113132

google/clif,google/clif,google/clif