Make channel binding data per-host

Previously, once you successfully generated channel binding data for one
host, it would be stored in self.cbt_struct, and self.cbt_binding_tried
would be set to True, preventing ever recalculating self.cbt_struct.
This means that once you used an HTTPKerberosAuth object to talk to one
host, it would keep sending that channel binding data to other hosts,
resulting in failures for servers that enforce channel binding.

Instead, store CBTs in a per-host dictionary as we do with GSSAPI
contexts.

requests_kerberos/kerberos_.py

@@ -183,8 +183,7 @@ def __init__(
 
         # Set the CBT values populated after the first response
         self.send_cbt = send_cbt
-        self.cbt_binding_tried = False
-        self.cbt_struct = None
+        self._cbts = {}
 
     def generate_request_header(self, response, host, is_preemptive=False):
         """
@@ -214,7 +213,7 @@ def generate_request_header(self, response, host, is_preemptive=False):
                 username=self.principal,
                 hostname=kerb_host,
                 service=self.service,
-                channel_bindings=self.cbt_struct,
+                channel_bindings=self._cbts.get(host, None),
                 context_req=gssflags,
                 protocol="kerberos",
             )
@@ -371,16 +370,18 @@ def handle_response(self, response, **kwargs):
         num_407s = kwargs.pop('num_407s', 0)
 
         # Check if we have already tried to get the CBT data value
-        if not self.cbt_binding_tried and self.send_cbt:
+        if self.send_cbt:
+            host = urlparse(response.url).hostname
             # If we haven't tried, try getting it now
-            cbt_application_data = _get_channel_bindings_application_data(response)
-            if cbt_application_data:
-                self.cbt_struct = spnego.channel_bindings.GssChannelBindings(
-                    application_data=cbt_application_data,
-                )
-
-            # Regardless of the result, set tried to True so we don't waste time next time
-            self.cbt_binding_tried = True
+            if host not in self._cbts:
+                cbt_application_data = _get_channel_bindings_application_data(response)
+                if cbt_application_data:
+                    self._cbts[host] = spnego.channel_bindings.GssChannelBindings(
+                        application_data=cbt_application_data,
+                    )
+                else:
+                    # Store None so we don't waste time next time
+                    self._cbts[host] = None
 
         if self.pos is not None:
             # Rewind the file position indicator of the body to where