Included __format__ in safe_attrs to support f-strings and calls such…

… as format()

rpyc/core/protocol.py

@@ -39,7 +39,7 @@ class PingError(Exception):
                     '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__',
                     '__rxor__', '__setitem__', '__setslice__', '__str__', '__sub__',
                     '__truediv__', '__xor__', 'next', '__length_hint__', '__enter__',
-                    '__exit__', '__next__', ]),
+                    '__exit__', '__next__', '__format__']),
     exposed_prefix="exposed_",
     allow_getattr=True,
     allow_setattr=False,