Force pip version 19

[vishalbollu]

Conda installs pip 20.* by default which uses the latest resolver. The new pip dependency resolver is more strict. For example:

ERROR: Cannot install keras==2.4.3, scikit-image==0.17.2, scikit-learn==0.23.2, scipy==1.5.2, statsmodels==0.12.0 and tensorflow-cpu==2.3.0 because these package versions have conflicting dependencies.

The conflict is caused by:
    The user requested scipy==1.5.2
    keras 2.4.3 depends on scipy>=0.14
    scikit-image 0.17.2 depends on scipy>=1.0.1
    scikit-learn 0.23.2 depends on scipy>=0.19.1
    statsmodels 0.12.0 depends on scipy>=1.1
    tensorflow-cpu 2.3.0 depends on scipy==1.4.1

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies

Pip 19 is more permissive. Force the default pip version to use version 19 for now.

---

checklist:

• run make test and make lint
• test manually (i.e. build/push all images, restart operator, and re-deploy APIs)
• update examples
• update docs and add any new files to summary.md (view in gitbook after merging)
• cherry-pick into release branches if applicable
• alert the dev team if the dev environment changed