Add markupsafe==2.0.1 to requirement.txt

This fixes a breaking change reported at
pallets/markupsafe#284.

The errors were showing up in the heroku logs thus:

    2022-02-21T00:47:47.248902+00:00 app[web.1]: from markupsafe import soft_unicode
    2022-02-21T00:47:47.248903+00:00 app[web.1]: ImportError: cannot import name 'soft_unicode' from 'markupsafe' (/app/.heroku/python/lib/python3.9/site-packages/markupsafe/__init__.py)

requirements.txt

@@ -4,6 +4,7 @@ dotmap~=1.3.26
 Flask~=1.1.2
 Flask-WTF
 gunicorn~=20.1.0
+markupsafe==2.0.1
 python-docx~=0.8.11
 python-dotenv~=0.19.2
 WTForms~=3.0.1