fix UnicodeDecodeError in PickerWidgetMixin (fixes #23055)

'%s' % _(whatever) will produce a str containing utf-8 characters, but
it will fail when interpolated with another unicode string, which
happend in:

  '%s %s' % (_('Format:'), self.options['format'])

as self.options['format'] is unicode.

src/authentic2/widgets.py

@@ -121,7 +121,7 @@ class PickerWidgetMixin(object):
 
         help_text = self.help_text
         if not help_text:
-            help_text = '%s %s' % (_('Format:'), self.options['format'])
+            help_text = u'%s %s' % (_('Format:'), self.options['format'])
 
         return mark_safe(BOOTSTRAP_INPUT_TEMPLATE % dict(
                     id=id,

tests/conftest.py

@@ -330,3 +330,11 @@ def assert_external_redirect(external_redirect):
         def check_location(response, default_return):
             assert response['Location'] == urlparse.urljoin('http://testserver/', default_return)
     return check_location
+
+
+@pytest.fixture
+def french_translation():
+    from django.utils.translation import activate, deactivate
+    activate('fr')
+    yield
+    deactivate()

tests/test_widget_datetimepicker.py

@@ -1,6 +1,13 @@
 from authentic2.widgets import DateTimeWidget, DateWidget, TimeWidget
 
-def test_widgets_init():
-    DateTimeWidget()
-    DateWidget()
-    TimeWidget()
+
+def test_widgets_init_and_render_no_locale():
+    DateTimeWidget().render('wt', '2019/12/12 12:34:34')
+    DateWidget().render('wt', '2019/12/12')
+    TimeWidget().render('wt', '12:34:34')
+
+
+def test_widgets_init_and_render_fr(french_translation):
+    DateTimeWidget().render('wt', '2019/12/12 12:34:34')
+    DateWidget().render('wt', '2019/12/12')
+    TimeWidget().render('wt', '12:34:34')