Fixes RuntimeWarning for improper bufferring on local connection

(cherry picked from commit 6b5a46c)

Author: MetricMike

mitogen/core.py

@@ -3860,7 +3860,7 @@ def _setup_importer(self):
         else:
             core_src_fd = self.config.get('core_src_fd', 101)
             if core_src_fd:
-                fp = os.fdopen(core_src_fd, 'rb', 1)
+                fp = os.fdopen(core_src_fd, 'rb', 0)
                 try:
                     core_src = fp.read()
                     # Strip "ExternalContext.main()" call from last line.