Enabling `tests/lang/ru/test_lemmatizer.py`, fixing a `unicode` issue (#3084)

<!--- Provide a general summary of your changes in the title. -->

## Description

See #3079. Here I'm merging into `develop` instead of `master`.

### Types of change
<!-- What type of change does your PR cover? Is it a bug fix, an enhancement
or new feature, or a change to the documentation? -->

Bug fix.

## Checklist
<!--- Before you submit the PR, go over this checklist and make sure you can
tick off all the boxes. [] -> [x] -->
- [x] I have submitted the spaCy Contributor Agreement.
- [x] I ran the tests, and all new and existing tests passed.
- [x] My changes don't require a change to the documentation, or if they do, I've added all required information.

spacy/lang/ru/lemmatizer.py

@@ -1,6 +1,9 @@
 # coding: utf8
+from __future__ import unicode_literals
+
 from ...symbols import ADJ, DET, NOUN, NUM, PRON, PROPN, PUNCT, VERB, POS
 from ...lemmatizer import Lemmatizer
+from ...compat import unicode_
 
 
 class RussianLemmatizer(Lemmatizer):
@@ -81,7 +84,7 @@ class RussianLemmatizer(Lemmatizer):
 
     @staticmethod
     def normalize_univ_pos(univ_pos):
-        if isinstance(univ_pos, str):
+        if isinstance(univ_pos, unicode_):
             return univ_pos.upper()
 
         symbols_to_str = {

spacy/tests/lang/ru/test_lemmatizer.py

@@ -39,7 +39,6 @@ def test_ru_lemmatizer_noun_lemmas(ru_lemmatizer, text, lemmas):
     assert sorted(ru_lemmatizer.noun(text)) == lemmas
 
 
-@pytest.mark.models("ru")
 @pytest.mark.parametrize(
     "text,pos,morphology,lemma",
     [