Add explicit dependency on netaddr

the dependencies file, causing failures on upgrade (and presumably for new
installs).

changelog.d/3488.bugfix

@@ -0,0 +1 @@
+Add explicit dependency on netaddr

synapse/python_dependencies.py

@@ -58,15 +58,13 @@ REQUIREMENTS = {
     "six": ["six"],
     "prometheus_client": ["prometheus_client"],
     "attr": ["attr"],
+    "netaddr>=0.7.18": ["netaddr"],
 }
 
 CONDITIONAL_REQUIREMENTS = {
     "web_client": {
         "matrix_angular_sdk>=0.6.8": ["syweb>=0.6.8"],
     },
-    "preview_url": {
-        "netaddr>=0.7.18": ["netaddr"],
-    },
     "email.enable_notifs": {
         "Jinja2>=2.8": ["Jinja2>=2.8"],
         "bleach>=1.4.2": ["bleach>=1.4.2"],