Catch StopIteration explicitly in group_by_object generator (#264)

Python 3.7 changed the behaviour of generators so that a StopIteration exception which bubbles up inside a generator is transformed into a RuntimeException. This means that when calling next() inside a generator, we have to explicitly catch the StopIteration and exit the function instead of relying on it to implicitly stop the generator. Otherwise the program will crash. Sigh.

For more information, see PEP 479.

https://www.python.org/dev/peps/pep-0479/
https://stackoverflow.com/questions/51700960/runtimeerror-generator-raised-stopiteration-every-time-i-try-to-run-app/51701040#51701040

pokedex/db/translations.py

@@ -377,7 +377,10 @@ def group_by_object(stream):
     Yields ((class name, object ID), (list of messages)) pairs.
     """
     stream = iter(stream)
-    current = next(stream)
+    try:
+        current = next(stream)
+    except StopIteration:
+        return
     current_key = current.cls, current.id
     group = [current]
     for message in stream: