Fix IP URL previews on Python 3

.coveragerc

@@ -9,4 +9,3 @@ source=
 
 [report]
 precision = 2
-ignore_errors = True

.gitignore

@@ -57,3 +57,4 @@ env/
 
 .vscode/
 .ropeproject/
+.coverage.*

changelog.d/4215.misc

@@ -0,0 +1 @@
+Getting URL previews of IP addresses no longer fails on Python 3.

synapse/config/repository.py

@@ -23,6 +23,10 @@
     "Missing netaddr library. This is required for URL preview API."
 )
 
+MISSING_HYPERLINK = (
+    "Missing hyperlink library. This is required for URL preview API."
+)
+
 MISSING_LXML = (
     """Missing lxml library. This is required for URL preview API.
 
@@ -151,6 +155,12 @@ def read_config(self, config):
             except ImportError:
                 raise ConfigError(MISSING_LXML)
 
+            try:
+                import hyperlink
+                hyperlink  # To stop unused lint.
+            except ImportError:
+                raise ConfigError(MISSING_HYPERLINK)
+
             try:
                 from netaddr import IPSet
             except ImportError: