Don't check expiry for creds that have none (fsspec#665)

Author: martindurant

gcsfs/credentials.py

@@ -174,9 +174,14 @@ def _credentials_valid(self, refresh_buffer):
             self.credentials.valid
             # In addition to checking current validity, we ensure that there is
             # not a near-future expiry to avoid errors when expiration hits.
-            and self.credentials.expiry
-            and (self.credentials.expiry - datetime.utcnow()).total_seconds()
-            > refresh_buffer
+            and (
+                (
+                    self.credentials.expiry
+                    and (self.credentials.expiry - datetime.utcnow()).total_seconds()
+                    > refresh_buffer
+                )
+                or not self.credentials.expiry
+            )
         )
 
     def maybe_refresh(self, refresh_buffer=300):