When installing a package I saw the notice that pip had an upgrade available. (Both 2.7.15 & 3.whateveriscurrent are installed) So I followed the instructions...

pip2 install --upgrade pip

Then when I tried doing this for pip3 afterward it was suddenly gone. Trying the python3 -m ensurepipwhatever command reports that it is installed, yet pip3 is missing, as is pip-3.x. Also, pip (no numeric suffix) --version reports that it's tied to python-2.7, so it's not just renamed.

The only way to fix this is to rm -rf /usr/lib/python* and reinstall them from source, which downgrades pip2 & pip3 to 9.x. Running pip3 install --upgrade pip FIRST will work. But, once again upgrading pip2 completely removes pip3.

Any ideas how to fix this?

Finally, I figured out a way to fix this...

pip2 install -U pip

followed by...

python3 -m pip install -U pip

There should now be both a pip2 and pip3 at the 10.0.1 version. The non-numbered pip command will be a copy of the last one that was upgraded. If a different one is desired, simply copy /usr/bin/pip2 or /usr/bin/pip3 to -> /usr/bin/pip as needed.

Why the upgrade process for pip2 removes any existing version of pip3 & pip-3.6 I still don't know. I just know that easy_install3 and python3 -m ensurepip etc. did not work to upgrade to a working copy of pip3 after pip2 was upgraded to 10.0.1.