Refactor pyfunc load_model to avoid deprecation warning

[dbczumar]

What changes are proposed in this pull request?

Calling mlflow.pyfunc.load_model currently produces a deprecation warning because its implementation invokes the deprecated mlflow.pyfunc.load_pyfunc function. I filed #1980 to remove load_pyfunc from the code base entirely; however, this requires a bit more testing / diligence than I have time to invest prior to the next release. Accordingly, it would be great to get this refactor merged quickly instead.

How is this patch tested?

Existing unit tests, manual tests

Release Notes

Is this a user-facing change?

• No. You can skip the rest of this section.
• Yes. Give a description of this change to be included in the release notes for MLflow users.

What component(s) does this PR affect?

• UI
• CLI
• API
• REST-API
• Examples
• Docs
• Tracking
• Projects
• Artifacts
• Models
• Scoring
• Serving
• R
• Java
• Python

How should the PR be classified in the release notes? Choose one:

• rn/breaking-change - The PR will be mentioned in the "Breaking Changes" section
• rn/none - No description will be included. The PR will be mentioned only by the PR number in the "Small Bugfixes and Documentation Updates" section
• rn/feature - A new user-facing feature worth mentioning in the release notes
• rn/bug-fix - A user-facing bug fix worth mentioning in the release notes
• rn/documentation - A user-facing documentation change worth mentioning in the release notes

[mparkhe]

looks good.