debian/rules: Use str.format over "+" for string concatenation.

debian/rules

@@ -73,7 +73,7 @@ override_dh_auto_clean:
 	@sed -i "s#%RECOMMENDS%#$(shell bin/diffoscope --list-debian-substvars | cut -d= -f2)#" debian/tests/control.tmp
 	@sed -i "s#%PYRECOMMENDS%#$(shell python3 -c "import distutils.core; \
 		setup = distutils.core.run_setup('setup.py'); \
-		print(', '.join(sorted(['python3-'+x for y in setup.extras_require.values() for x in y])))" \
+		print(', '.join(sorted(['python3-{}'.format(x) for y in setup.extras_require.values() for x in y])))" \
 	)#" debian/tests/control.tmp
 	@sed -i "s,python3-python-debian,python3-debian," debian/tests/control.tmp
 	@sed -i "s,python3-rpm-python,python3-rpm," debian/tests/control.tmp