Improve --patchbot command

src/bin/sage

@@ -276,7 +276,26 @@ fi
 # an unclobbered environment before testing unsafe tickets.
 if [ "$1" = '-patchbot' -o "$1" = "--patchbot" ]; then
     shift
+    # We ask the Python from Sage where the patchbot is installed.
+    # We set PYTHONPATH to that directory such that the system Python
+    # should also find the sage_patchbot package.
+    cmd='import sage_patchbot as p; import os; print(os.path.dirname(p.__path__[0]))'
+    export PYTHONPATH=`"$SAGE_ROOT/sage" --python -c "$cmd"`
+    if [ -z "$PYTHONPATH" ]; then
+        # Something went wrong, assume that the patchbot is not installed
+        echo >&2 "Error: cannot find installation path for sage_patchbot"
+        echo >&2 "See https://wiki.sagemath.org/buildbot for instructions"
+        exit 1
+    fi
+
+    # Try "python2.7", then "python2", then "python"
+    shopt -s execfail  # Do not exit if "exec" fails
+    exec python2.7 -m sage_patchbot.patchbot "$@"
+    exec python2 -m sage_patchbot.patchbot "$@"
     exec python -m sage_patchbot.patchbot "$@"
+    echo >&2 "Error: cannot find a suitable Python program."
+    echo >&2 "The SageMath patchbot requires a system Python installation."
+    exit 127
 fi
 
 # Check for '--upgrade' before sourcing sage-env: the top-level "make"