Make next_sequence_number private (#62)

We should only use this in the table module

Author: Fokko

pyiceberg/table/__init__.py

@@ -533,7 +533,7 @@ def location(self) -> str:
     def last_sequence_number(self) -> int:
         return self.metadata.last_sequence_number
 
-    def next_sequence_number(self) -> int:
+    def _next_sequence_number(self) -> int:
         return INITIAL_SEQUENCE_NUMBER if self.format_version == 1 else self.last_sequence_number + 1
 
     def new_snapshot_id(self) -> int: