slapos_web_deploy: upgrade pip before using it

This is particularly needed for old OS with python2: very old versions of
pip are trying to install latest version of eggs no matter their python2
compatibility whereas pip 20 is making sure to install only python2
compatible eggs.

master/bt5/slapos_web_deploy/PathTemplateItem/web_page_module/deploy_base_setup.txt

@@ -129,9 +129,11 @@ is_package_installed ansible || install_package ansible
 
 if ! is_ubuntu || [[ $DISTRO = wheezy || $DISTRO == jessie || $DISTRO == stretch || $DISTRO == xenial || $DISTRO == bionic ]]; then
   is_package_installed python-pip || install_package python-pip
+  pip install --upgrade "pip < 21.0"
 else
   # Ansible is using python3 now
   is_package_installed python3-pip || install_package python3-pip
+  pip3 install --upgrade pip
 fi
 
 if is_ubuntu && [[ $DISTRO == focal || $DISTRO == buster ]]; then