Ignore gevent when using TestContext if not installed
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.
Related: !600 (merged)
Edited by Anton Joubert