workaround new limitation in requests

newer requests do not expose the internal SSL socket object so we
cannot verify certificates. there was work to allow custom
verification routines which we could use, but this never finished:

https://github.com/shazow/urllib3/pull/257

so right now, just treat missing socket information as if the cert was
missing.

Closes: #76

linkcheck/checker/httpurl.py

@@ -194,6 +194,10 @@ class HttpUrl (internpaturl.InternPatternUrl, proxysupport.ProxySupport):
         """Get raw SSL socket."""
         assert self.scheme == u"https", self
         raw_connection = self.url_connection.raw._connection
+        if not raw_connection:
+            # this happens with newer requests versions:
+            # https://github.com/linkcheck/linkchecker/issues/76
+            return None
         if raw_connection.sock is None:
             # sometimes the socket is not yet connected
             # see https://github.com/kennethreitz/requests/issues/1966
@@ -204,7 +208,10 @@ class HttpUrl (internpaturl.InternPatternUrl, proxysupport.ProxySupport):
         """Add SSL cipher info."""
         if self.scheme == u'https':
             sock = self._get_ssl_sock()
-            if hasattr(sock, 'cipher'):
+            if not sock:
+                log.debug(LOG_CHECK, "cannot extract SSL certificate from connection")
+                self.ssl_cert = None
+            elif hasattr(sock, 'cipher'):
                 self.ssl_cert = sock.getpeercert()
             else:
                 # using pyopenssl