Use user mode for pip upgrade

This is default for some platforms, but not everywhere, so use it
explicitly.

This should fix the error in Travis CI S/390 build.

build/tools/httpbin.sh

@@ -48,7 +48,7 @@ httpbin_launch() {
             #
             # However don't upgrade to a version which is too new because then
             # it may not support Python version that we actually have.
-            python3 -m pip install --upgrade pip==20.0.1
+            python3 -m pip install --user --upgrade pip==20.0.1
             ;;
     esac