Make netifaces optional

netifaces is a binary Python extension. Outside of the installer, it's
only used by CheckedIPAddress.get_matching_interface, which is only
called from installer code.

Make the import of netifaces optional to reduce the amount of
dependencies for PyPI package use case. Binary extensions are especially
annoying, because they depend on shared libraries, compiler, and header
files to be present.

Related: https://pagure.io/freeipa/issue/6468
Signed-off-by: Christian Heimes <cheimes@redhat.com>
Reviewed-By: Oleg Kozlov <okozlov@redhat.com>

ipapython/setup.py

@@ -43,11 +43,12 @@ if __name__ == '__main__':
             # "ipalib",  # circular dependency
             "ipaplatform",
             "netaddr",
-            "netifaces",
             "python-ldap",
             "six",
         ],
         extras_require={
             "install": ["dbus-python"],  # for certmonger
+            # CheckedIPAddress.get_matching_interface
+            "netifaces": ["netifaces"],
         },
     )