docs: Call reformat-news.py with $(PYTHON)

This is the only Python script which we invoke directly, which
works pretty fine in general but becomes a problem if the user
has explicitly overridden Python binary detection at configure
time.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

Makefile.am

@@ -57,7 +57,7 @@ NEWS: \
 	    $(srcdir)/docs/news.xml \
 	  >$@-tmp \
 	    || { rm -f $@-tmp; exit 1; }; \
-	  $(srcdir)/docs/reformat-news.py $@-tmp >$@ \
+	  $(PYTHON) $(srcdir)/docs/reformat-news.py $@-tmp >$@ \
 	    || { rm -f $@-tmp; exit 1; }; \
 	  rm -f $@-tmp; \
 	fi