fish: add pip upgrade script

fish/functions/pip_upgrade.fish

@@ -0,0 +1,11 @@
+function pip3_upgrade --description "Upgrade python packages (pip3)"
+    set --local py_pkgs_outdated (pip3 list --outdated | awk 'NR > 2 {print $1}')
+
+    if test (count $py_pkgs_outdated) = 0
+        echo "All packages up-to-date."
+    else
+        for py_pkg in $py_pkgs_outdated
+            pip3 install $py_pkg -U
+        end
+    end
+end