build/bin/sage-bootstrap-python: Test that python has argparse

build/bin/sage-bootstrap-python

@@ -17,6 +17,9 @@ fi
 # So it needs to find a python that has the urllib module.
 # For example, on Debian buster, the python3-minimal package does NOT provide it.
 #
+# Also, Trac #20023 removed the vendored argparse library from sage_bootstrap,
+# so we test that python is new enough (>= 2.7) to run it.
+#
 # See https://trac.sagemath.org/ticket/29090
 
 # Trac #29890: Our first choice is "python", not "python3". This is to avoid
@@ -41,7 +44,7 @@ PYTHONS="python python3 python3.8 python3.7 python2.7 python3.6 python2"
 for PY in $PYTHONS; do
     PYTHON="$(PATH="$SAGE_ORIG_PATH" command -v $PY)"
     if [ -n "$PYTHON" ]; then
-        if "$PYTHON" -c "import urllib; from hashlib import sha1; from os import listdir; listdir(\"$(pwd)\");" 2>/dev/null; then
+        if "$PYTHON" -c "import argparse; import urllib; from hashlib import sha1; from os import listdir; listdir(\"$(pwd)\");" 2>/dev/null; then
             exec "$PYTHON" "$@"
         fi
     fi