Make vocab update in get_docs deterministic

The attribute `DocBin.strings` is a set. In `DocBin.get_docs`
a given vocab is updated by iterating over this set.
Iteration over a python set produces an arbitrary ordering,
therefore vocab is updated non-deterministically.

When training (fine-tuning) a spacy model, the base model's
vocabulary will be updated with the new vocabulary in the
training data in exactly the way described above. After
serialization, the file `model/vocab/strings.json` will
be sorted in an arbitrary way. This prevents reproducible
model training.

spacy/tokens/_serialize.py

@@ -124,7 +124,7 @@ class DocBin:
 
         DOCS: https://spacy.io/api/docbin#get_docs
         """
-        for string in self.strings:
+        for string in sorted(self.strings):
             vocab[string]
         orth_col = self.attrs.index(ORTH)
         for i in range(len(self.tokens)):