fix dep versions

example/fake_api/flask_api.py

         """
         delete user
         """
-        return NoContentSchema()
+        resp = flask.Response(
+            status=204,
+        )
+        # See tests.func.fake_api.test_flask.test_func_flask_fake_api
+        del resp.headers['Content-Type']
+        return resp
 
     def bind(self, app: flask.Flask):
         app.add_url_rule('/about',

setup.py

 here = path.abspath(path.dirname(__file__))
 
 install_requires = [
-    'marshmallow',
+    'marshmallow >2.0.0,<3.0.0a1',
     'apispec',
     'multidict'
 ]