Checking for compatibility with SimPy 4

[eileen-kuehn]

SimPy came up with version 4 around 2 months ago. There were some changes, most importantly they now only work with Python 3.6+. Further, the following changes are mentioned in the documentation:

• BaseEnvironment has been dropped, Environment should be used instead
• Returning from Process generators now by return keyword: usage of Environment.exit(value) and raise StopProcess(value) should be replaced by return

Source: https://simpy.readthedocs.io/en/latest/topical_guides/porting_from_simpy3.html

We should double check if everything works as expected.

[maxfischer2781]

To map what we currently have in usim to SimPy 4.0:

• We never added BaseEnvironment. It is not listed as unsupported in the docs either. Nothing to do.

• We do have Environment.exit and StopProcess. Both are listed in the docs as well. We natively support return already. Need to remove both exit and StopProcess.