Deal with both old and new locations of Var of docplex

[t-imamichi]

Summary

Follow-up of #1520
Backport of qiskit-community/qiskit-optimization#23

Details and comments

I replaced mp.linear.Var with mp.dvar.Var to be compatible with the latest docplex 2.20.204.
mp.linear.Var was used until 2.15.194. So, I modified the code to work with both styles.

I add a version pin docplex==2.15.194 for macOS because docplex does not work without cplex on macOS.
See #24 for details.

[woodsp-ibm]

I see 2.16.196 was released in early Nov last year - so its not that old. If the two classes are compatible in terms of internals could we do code that imported the Var from the new location and if that failed try the location where we imported it from before? This would give a broader compatibility if its reasonably doable.

-- See discussion, on same topic, in qiskit-community/qiskit-optimization#23

[t-imamichi]

I would like to include it as part of Aqua 0.8.2 because some users may use docplex 2.15 or older. We need to discuss it as well as #1530

[woodsp-ibm]

For now perhaps we should add the equivalent setup.py version pin of docplex as was done in the stable branch for the 0.8.2. release. This can be revisited if some better solution is found but at least then it will compatible to with what has just been released.

[t-imamichi]

Yes, it might be good to pin docplex==2.15 until a bug fix of docplex is released so that docplex works without cplex.

[t-imamichi]

This requires #1533 to avoid issues caused by astroid 2.5.

Update:
I see pylint 2.6.2 fixes the lint issues.