tox: limit attrs, for pytest

File ".../_pytest/tmpdir.py", line 35, in TempPathFactory
    lambda p: Path(os.path.abspath(six.text_type(p)))
TypeError: attrib() got an unexpected keyword argument 'convert'

tox.ini

@@ -21,6 +21,7 @@ deps =
         pg: psycopg2
         pylint<1.8
         pylint-django<0.8.1
+        attrs<19.2
         pytest-cov
         pytest-random
         pytest-mock