mongoengine: Incompatibilities with pymongo 4.0

There is a breaking change in pymongo 4.0 that was released very recently

It fails whenever we save an instance right now

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/usr/local/lib/python3.8/site-packages/mongoengine/document.py:407: in save
    self.ensure_indexes()
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:897: in ensure_indexes
    collection = cls._get_collection()
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:220: in _get_collection
    cls.ensure_indexes()
/usr/local/lib/python3.8/site-packages/mongoengine/document.py:900: in ensure_indexes
    if not collection.is_mongos and collection.read_preference > 1:
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self = Collection(Database(MongoClient(host=['mongo:27017'], document_class=dict, tz_aware=False, connect=True, read_preference=Primary()), 'cpreview_test'), 'test_document.is_mongos')
    def __bool__(self):
>       raise NotImplementedError("Collection objects do not implement truth "
                                  "value testing or bool(). Please compare "
                                  "with None instead: collection is not None")
E       NotImplementedError: Collection objects do not implement truth value testing or bool(). Please compare with None instead: collection is not None
/usr/local/lib/python3.8/site-packages/pymongo/collection.py:300: NotImplementedError