Don't use broken pip version 20.0

Pip continues to impress with version 20 being completely broken and not
running at all.

Try the next one, which is supposed to fix this bug.

build/tools/httpbin.sh

@@ -43,7 +43,7 @@ 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
+            python3 -m pip install --upgrade pip==20.0.1
             ;;
     esac