Docplex is not compatible with Python 3.9

[t-imamichi]

Information

• Qiskit Optimization version: NA
• Python version: Python 3.9.1
• Operating system: macOS 11.2

What is the current behavior?

The latest Docplex 2.20.204 does not work with Python 3.9

Related to #24

Steps to reproduce the problem

from docplex.mp.model import Model
a = Model()
x = a.binary_var()
print(a.add_constraint(x <= 1))

Python 3.8

x1 <= 1

Python 3.9

Traceback (most recent call last):
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 290, in get_cplex_module
    import cplex  #@UnresolvedImport
ModuleNotFoundError: No module named 'cplex'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/ima/tasks/1_2021/qiskit/aqua/tmp/c.py", line 2, in <module>
    a = Model()
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/model.py", line 417, in __init__
    self._environment = self._make_environment()
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/model.py", line 181, in _make_environment
    env = Environment.get_default_env()
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 427, in get_default_env
    Environment._default_env = Environment.make_new_configured_env()
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 422, in make_new_configured_env
    return Environment(start_auto_configure=True)
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 84, in __init__
    self.auto_configure(logger=logger)
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 220, in auto_configure
    self.check_cplex(logger=logger)
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 328, in check_cplex
    cplex = self.get_cplex_module(logger=logger)
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 314, in get_cplex_module
    cplex = load_cplex_from_cos_root(loc) if loc else None
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 286, in load_cplex_from_cos_root
    return load_cplex(full_path, version=version)
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/docplex/mp/environment.py", line 261, in load_cplex
    raise FileNotFoundError("Could not load module from %s" % module_location)
FileNotFoundError: Could not load module from /Applications/CPLEX_Studio201/cplex/python/3.9/x86-64osx/cplex/__init__.py

What is the expected behavior?

Suggested solutions

We should configure setup.py and requirements.txt so that Python 3.9 does not install docplex.

[t-imamichi]

I merge this issue with #24