add a new line to setup.py

[wangger]

add dill==0.3.4 to setup.py

[Hanrui-Wang]

Thanks for the pr! Please specify the reason for dill==0.3.4 insteal of 0.3.6

[wangger]

When we run the code on the server, we might have the error, "TypeError: cannot pickle '_abc._abc_data' object". There is a discussion about this error, uqfoundation/dill#332.
By downgrading the version of dill from 0.3.6 to 0.3.4, the problem was solved. I guess dill 0.3.5 might also work but I did not verify it.

[Hanrui-Wang]

Got it. thank you! merged