setup.py: Add mising dependency.

This was indirectly satisfied by cryptography depending on ipaddress, so
no functional change is expected. This is just to be pedantic.

CHANGES.txt

+0.9.? (????-??-??)
+==================
+* Explicitly depend on ipadress module on python 2.7 .
+
 0.9.13 (2021-12-22)
 ===================
 * Improve behaviour on computers which are frequently suspended. This should avoid missing certificate renewal deadlines.

setup.py

@@ -53,6 +53,7 @@ setup(
     'pyOpenSSL>=18.0.0', # ...certificate chain validation
     'pem>=18.2.0', # Parse PEM files
     'PyJWT', # CORS token signature
+    'ipaddress; python_version<"3"',
   ],
   zip_safe=True,
   entry_points={