Replace %s rather than fallback to defaults

3a0add4b6e caused existing configs to fall
back to defaults. But since we know exactly how to fix the user config
so that the behavior doesn't change, we should do so, while still
warning that use of %s is deprecated.

Also fixes a bug where we tried to look for %s in None.

pelican/tests/test_settings.py

@@ -9,6 +9,7 @@ from sys import platform
 
 
 from pelican.settings import (DEFAULT_CONFIG, DEFAULT_THEME,
+                              _printf_s_to_format_field,
                               configure_settings, handle_deprecated_settings,
                               read_settings)
 from pelican.tests.support import unittest
@@ -168,6 +169,14 @@ class TestSettingsConfiguration(unittest.TestCase):
 
         self.assertRaises(Exception, configure_settings, settings)
 
+    def test__printf_s_to_format_field(self):
+        for s in ('%s', '{%s}', '{%s'):
+            option = 'foo/{}/bar.baz'.format(s)
+            result = _printf_s_to_format_field(option, 'slug')
+            expected = option % 'qux'
+            found = result.format(slug='qux')
+            self.assertEqual(expected, found)
+
     def test_deprecated_extra_templates_paths(self):
         settings = self.settings
         settings['EXTRA_TEMPLATES_PATHS'] = ['/foo/bar', '/ha']