Fix bug that prevents loading old models (#3359)

* fix loading old gensim model by new model

* Revert "fix loading old gensim model by new model"

This reverts commit d398e92.

* fix loading old gensim model by new model

* Update word2vec.py

* Update word2vec.py

* Update word2vec.py

Co-authored-by: Michael Penkov <m@penkov.dev>

gensim/models/word2vec.py

@@ -200,6 +200,9 @@
 from gensim.models.keyedvectors import KeyedVectors, pseudorandom_weak_vector
 from gensim import utils, matutils
 
+# This import is required by pickle to load models stored by Gensim < 4.0, such as Gensim 3.8.3.
+from gensim.models.keyedvectors import Vocab  # noqa
+
 from smart_open.compression import get_supported_extensions
 
 logger = logging.getLogger(__name__)