setup.py: Pin werkzeug<2.1

`utils/hash.py` uses `pbkdf2_bin` from werkzeug, which will
be removed in 2.1.
Pin the version until a fix is made.

See error:
```
isso/utils/hash.py:71: DeprecationWarning: 'pbkdf2_bin' is deprecated and will be removed in Werkzeug 2.1. Use 'hashlib.pbkdf2_hmac()' instead.
    return pbkdf2(val, self.salt, self.iterations, self.dklen, self.func)
```

setup.py

@@ -6,7 +6,7 @@
 from setuptools import setup, find_packages
 
 requires = ['itsdangerous', 'Jinja2', 'misaka>=2.0,<3.0', 'html5lib',
-            'werkzeug>=1.0', 'bleach', 'Flask-Caching>=1.9', 'Flask']
+            'werkzeug>=1.0,<2.1', 'bleach', 'Flask-Caching>=1.9', 'Flask']
 tests_require = ['pytest', 'pytest-cov']
 
 # https://packaging.python.org/en/latest/guides/making-a-pypi-friendly-readme/