optimize runtime

A `RatingRow`'s `id` and `updated` are never used, but creating them
a million+ times is quite expensive, so initializing them with `None`
saves a lot of time.
`dataclasses`' `fields` function is also quite expensive; loading the
fields from a row directly saves a lot of CPU cycles.

unwind/imdb_import.py

@@ -83,7 +83,7 @@ class RatingRow:
 
     @classmethod
     def from_row(cls, row):
-        inst = cls(*(f.type(r) for f, r in zip(fields(cls), row)))
+        inst = cls(tconst=row[0], averageRating=float(row[1]), numVotes=int(row[2]))
         assert inst.tconst != r"\N"
         return inst
 
@@ -91,6 +91,8 @@ class RatingRow:
         return Movie(
             imdb_id=self.tconst,
             score=score_from_imdb_rating(self.averageRating),
+            updated=None,  # optimization: skip default factory
+            id=None,  # optimization: skip default factory
         )