install_pip: Use packaged pip on Fedora

This uses the python3-pip package for Fedora but maintains the status
quo for existing distributions (i.e. for Suse we run get-pip.py but
don't uninstall, and for everything else we uninstall python3-pip and
run get-pip.py to be running the latest pip).

As noted inline, installing get-pip.py over Fedora 34's package no
longer works, and likely won't ever work again.  Unlike the LTS
distributions, the Fedora pip should be more up-to-date, so I think
it's best we just avoid any package overwrites.

Change-Id: I84129aadfcf585bb150a3daa39616246d3d84bbd

Author: ianw

tools/install_pip.sh

@@ -126,19 +126,31 @@ if [[ -n $PYPI_ALTERNATIVE_URL ]]; then
     configure_pypi_alternative_url
 fi
 
-# Eradicate any and all system packages
-
-# Python in fedora/suse depends on the python-pip package so removing it
-# results in a nonfunctional system. pip on fedora installs to /usr so pip
-# can safely override the system pip for all versions of fedora
-if ! is_fedora  && ! is_suse; then
+if is_fedora && [[ ${DISTRO} == f* ]]; then
+    # get-pip.py will not install over the python3-pip package in
+    # Fedora 34 any more.
+    #  https://bugzilla.redhat.com/show_bug.cgi?id=1988935
+    #  https://github.com/pypa/pip/issues/9904
+    # You can still install using get-pip.py if python3-pip is *not*
+    # installed; this *should* remain separate under /usr/local and not break
+    # if python3-pip is later installed.
+    # For general sanity, we just use the packaged pip.  It should be
+    # recent enough anyway.
+    install_package python3-pip
+elif is_fedora || is_suse; then
+    # Python in suse/centos depends on the python-pip package; because
+    # of the split "system-python" uninstalling python3-pip also
+    # uninstalls the user python3 package which is bad and leaves us
+    # without a python to use.  Just install over.
+    install_get_pip
+else
+    # Remove packaged pip, and install the latest upstream.
     if is_package_installed python3-pip ; then
         uninstall_package python3-pip
     fi
+    install_get_pip
 fi
 
-install_get_pip
-
 set -x
 
 # Note setuptools is part of requirements.txt and we want to make sure