Fix check for .mo files in translation directories

In Python 2 the `filter()` worked because it returned a list, but in
Python 3 it's a generator which is never falsy.

That aside, using `any()` there is much cleaner (filter-with-lambda was
never pythonic anyway).

Author: ThiefMaster

flask_babel/__init__.py

@@ -151,7 +151,7 @@ def list_translations(self):
                 if not os.path.isdir(locale_dir):
                     continue
 
-                if filter(lambda x: x.endswith('.mo'), os.listdir(locale_dir)):
+                if any(x.endswith('.mo') for x in os.listdir(locale_dir)):
                     result.append(Locale.parse(folder))
 
         # If not other translations are found, add the default locale.

tests/test_integration.py

@@ -22,6 +22,9 @@ def test_no_request_context():
 def test_multiple_directories():
     """
     Ensure we can load translations from multiple directories.
+
+    This also ensures that directories without any translation files
+    are not taken into account.
     """
     b = babel.Babel()
     app = flask.Flask(__name__)