Adopt to mlflow >= 1.9.0

[HiromuHota]

Description of the problems or issues

Is your pull request related to a problem? Please describe.
A clear and concise description of what the problem is.

See #461.

Does your pull request fix any issue.

Fix #461.

Description of the proposed changes

A clear and concise description of what you propose.

In Fonduer, FonduerModel._process and FonduerModel._classify have been spied and mocked, respectively, for testing purpose.
However, as of MLflow v1.9.0, a PyFunc model (e.g., HardwareFonduerModel) is now wrapped by mlflow.pyfunc.PyFuncModel (mlflow/mlflow#2920) and can only be accessed by its _model_impl attribute.
This PR spies and mocks on _model_impl attribute if mlflow>=1.9.0.

Test plan

A clear and concise description of how you test the new changes.

No new test plan. I just fixed existing tests so they work with both mlflow<1.9.0 and >=1.9.0.

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the CHANGELOG.rst accordingly.

[codecov-commenter]

Codecov Report

Merging #463 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #463   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files          88       88           
  Lines        4563     4563           
  Branches      837      837           
=======================================
  Hits         3800     3800           
  Misses        572      572           
  Partials      191      191           

Flag	Coverage Δ
#unittests	83.27% <ø> (ø)

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)

[senwu]

LGTM