Resolve reference to null value doc build issue, add __all__ dict

Author: rgommers

spec/API_specification/dataframe_api/__init__.py

@@ -12,6 +12,16 @@
 from ._types import DType
 
 
+__all__ = [
+    "__dataframe_api_version",
+    "column_from_sequence",
+    "concat",
+    "dataframe_from_dict",
+    "isnull",
+    "null",
+]
+
+
 __dataframe_api_version__: str = "YYYY.MM"
 """
 String representing the version of the DataFrame API specification to which

spec/API_specification/dataframe_api/column_object.py

@@ -457,7 +457,7 @@ def unique_indices(self, *, skip_nulls: bool = True) -> Column[int]:
         """
         ...
 
-    def fill_nan(self, value: float | null, /):
+    def fill_nan(self, value: float | 'null', /):
         """
         Fill floating point ``nan`` values with the given fill value.
 

spec/API_specification/dataframe_api/dataframe_object.py

@@ -707,7 +707,7 @@ def isnan(self) -> DataFrame:
         """
         ...
 
-    def fill_nan(self, value: float | null, /):
+    def fill_nan(self, value: float | 'null', /):
         """
         Fill ``nan`` values with the given fill value.