Use markupsafe constraint in extras

Author: bgyori

setup.py

@@ -36,7 +36,8 @@ def main():
                       'plot': ['matplotlib'],
                       'isi': ['nltk<3.6', 'unidecode'],
                       'api': ['flask<2.0', 'flask_restx<0.4', 'flask_cors',
-                              'docstring-parser', 'gunicorn'],
+                              'docstring-parser', 'gunicorn',
+                              'markupsafe<2.1.0'],
                       'sklearn_belief': ['scikit-learn'],
                       'owl': ['pronto'],
                       'tests': ['pytest', 'pytest-cov'],