In 9.5.0rc1 testing we see that using the test context causes

`ModuleNotFoundError: No module named 'gevent'`

if user doesn't have gevent installed.  The gevent package is an optional
dependency for PyTango, so if a user isn't doing anything
with gevent, then it should not be required.

The `switch_existing_global_executors_to_thread` function has been updated to perform
this check.