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():

tests/scanner.TestCase

@@ -508,13 +508,13 @@ class Test_SignatureDataController(unittest.TestCase):
     # check_last_updated
     def test_check_last_updated_ok(self):
         sdc = fdroidserver.scanner.SignatureDataController('nnn', 'fff.yml', 'https://example.com/test.json')
-        sdc.data['last_updated'] = datetime.now().astimezone().isoformat()
+        sdc.data['last_updated'] = datetime.utcnow().isoformat()
         sdc.check_last_updated()
 
     def test_check_last_updated_exception_cache_outdated(self):
         sdc = fdroidserver.scanner.SignatureDataController('nnn', 'fff.yml', 'https://example.com/test.json')
         sdc.cache_duration = timedelta(days=7)
-        sdc.data['last_updated'] = (datetime.now().astimezone() - timedelta(days=30)).isoformat()
+        sdc.data['last_updated'] = (datetime.utcnow() - timedelta(days=30)).isoformat()
         with self.assertRaises(fdroidserver.scanner.SignatureDataOutdatedException):
             sdc.check_last_updated()