avoid running into native-date object issue

Seem I ran into this issue: https://bugs.python.org/issue47228  This
change tries to fix it by using utcnow insteas of astimezone.

fdroidserver/scanner.py

@@ -114,7 +114,7 @@ def get_embedded_classes(apkfile, depth=0):
 
 def _datetime_now():
     """Get datetime.now(), using this funciton allows mocking it for testing."""
-    return datetime.now().astimezone()
+    return datetime.utcnow()
 
 
 def _scanner_cachedir():