plpython: Fix sed expression in python3 build

The old expression sed 's,$(srcdir),python3,' would normally resolve
as sed 's,.,python3,', which is not really what we wanted.  While it
doesn't actually break anything right now, it's still wrong, so put in
a bit more work to make it more robust.

src/pl/plpython/Makefile

@@ -131,7 +131,7 @@ prep3:
 	      -e "s/LANGUAGE plpython2u/LANGUAGE plpython3u/g" \
 	      -e "s/EXTENSION plpythonu/EXTENSION plpython3u/g" \
 	      -e "s/EXTENSION plpython2u/EXTENSION plpython3u/g" \
-	    $$file >`echo $$file | sed 's,$(srcdir),python3,'` || exit; \
+	    $$file >`echo $$file | sed 's,^.*\(/[^/][^/]*/[^/][^/]*\)$$,python3\1,'` || exit; \
 	done
 
 clean3: