PYTHON-1407 Ignore non-readable servers' session timeout.

Author: ShaneHarvey

pymongo/topology_description.py

@@ -95,14 +95,14 @@ def __init__(self,
  # of known ServerType. If any ServerDescription of known ServerType has
  # a null logicalSessionTimeoutMinutes, then
  # TopologyDescription.logicalSessionTimeoutMinutes MUST be set to null.
- known = self.known_servers
- if not known:
+ readable = [s for s in self.known_servers if s.is_readable]
+ if not readable:
  self._ls_timeout_minutes = None
- elif any(s.logical_session_timeout_minutes is None for s in known):
+ elif any(s.logical_session_timeout_minutes is None for s in readable):
  self._ls_timeout_minutes = None
  else:
  self._ls_timeout_minutes = min(s.logical_session_timeout_minutes
- for s in known)
+ for s in readable)
 
  def check_compatible(self):
  """Raise ConfigurationError if any server is incompatible.