Lock Sphinx to v3.3.1

Because:
- A bug in Sphinx 3.4 seems to cause the docs search function to stop
  working: sphinx-doc/sphinx#8623
- Locking on to the 3.3.1 version is an acceptable workaround for now..

Fixes #2270

requirements/base.txt

@@ -12,7 +12,7 @@ networkx>=2.2,<2.3
 Pillow>3.3.2,<8.1
 pyrad==2.1
 python-ldap==3.0.0 # optional for LDAP authentication, requires libldap (OpenLDAP) to build
-sphinx>=1.8.0
+sphinx==3.3.1
 feedparser>=5.2.1,<5.3
 markdown==2.5.1
 dnspython==1.15.0