Update pip version to > 19.0

[brandjon]

This is to match tensorflow's requirement.

Current pip version is 9.0.3. (Looks like pip recently switched the versioning scheme to use the last two digits of the year.)

[bshashank]

Given the desire to move away from using par files (#184), are we going to update pip by updating the vendored third_party directory like it was suggested in #81?

[brandjon]

I'm just intending to bump the version number and confirm it fixes a repro that's been reported with tensorflow. I'd like to eliminate the circular par dependency (#184) but it's not a blocker and not urgent.

[thundergolfer]

#412 upgraded pip to 21.x.x for the pip_install repo rule, so I think we can now close this issue 🙂.