Simplify pip install

The uninstall here has been around since
Ibb4b42119dc2e51577c77bbbbffb110863e5324d.  At the time, there might
have been conflicts between packaged and installed pip.  We don't need
it today; get-pip.py keeps itself separate enough in /usr/local on all
platforms.  Thus we can also remove the suse/centos special-casing.

python3-pip is in the RPM list so we don't need to
re-install for Fedora.

Add a note on why we are over-installing pip.

Remove some old setuptools workarounds that are commented out.

Change-Id: Ie3cb81a8ff71cf4b81e23831c380f83b0381de71

Author: ianw

stack.sh

@@ -752,7 +752,19 @@ git_clone $REQUIREMENTS_REPO $REQUIREMENTS_DIR $REQUIREMENTS_BRANCH
 echo_summary "Installing package prerequisites"
 source $TOP_DIR/tools/install_prereqs.sh
 
-# Configure an appropriate Python environment
+# Configure an appropriate Python environment.
+#
+# NOTE(ianw) 2021-08-11 : We install the latest pip here because pip
+# is very active and changes are not generally reflected in the LTS
+# distros.  This often involves important things like dependency or
+# conflict resolution, and has often been required because the
+# complicated constraints etc. used by openstack have tickled bugs in
+# distro versions of pip.  We want to find these problems as they
+# happen, rather than years later when we try to update our LTS
+# distro.  Whilst it is clear that global installations of upstream
+# pip are less and less common, with virtualenv's being the general
+# approach now; there are a lot of devstack plugins that assume a
+# global install environment.
 if [[ "$OFFLINE" != "True" ]]; then
     PYPI_ALTERNATIVE_URL=${PYPI_ALTERNATIVE_URL:-""} $TOP_DIR/tools/install_pip.sh
 fi

tools/install_pip.sh

@@ -111,14 +111,6 @@ function configure_pypi_alternative_url {
 
 }
 
-# Setuptools 8 implements PEP 440, and 8.0.4 adds a warning triggered any time
-# pkg_resources inspects the list of installed Python packages if there are
-# non-compliant version numbers in the egg-info (for example, from distro
-# system packaged Python libraries). This is off by default after 8.2 but can
-# be enabled by uncommenting the lines below.
-#PYTHONWARNINGS=$PYTHONWARNINGS,always::RuntimeWarning:pkg_resources
-#export PYTHONWARNINGS
-
 # Show starting versions
 get_versions
 
@@ -135,19 +127,9 @@ if is_fedora && [[ ${DISTRO} == f* ]]; then
     # 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
+    # recent enough anyway.  This is included via rpms/general
+    continue
 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