TYP: move IndexLabel to pandas._typing

This eliminates repetition of the type annotations
for index label in multiple places.

Author: ivanovmg

pandas/_typing.py

@@ -15,6 +15,7 @@
     List,
     Mapping,
     Optional,
+    Sequence,
     Type,
     TypeVar,
     Union,
@@ -82,6 +83,7 @@
 
 Axis = Union[str, int]
 Label = Optional[Hashable]
+IndexLabel = Optional[Union[bool, str, Sequence[Label]]]
 Level = Union[Label, int]
 Ordered = Optional[bool]
 JSONSerializable = Optional[Union[PythonScalar, List, Dict]]

pandas/core/generic.py

@@ -40,6 +40,7 @@
     CompressionOptions,
     FilePathOrBuffer,
     FrameOrSeries,
+    IndexLabel,
     JSONSerializable,
     Label,
     Level,
@@ -3160,7 +3161,7 @@ def to_csv(
         columns: Optional[Sequence[Label]] = None,
         header: Union[bool_t, List[str]] = True,
         index: bool_t = True,
-        index_label: Optional[Union[bool_t, str, Sequence[Label]]] = None,
+        index_label: IndexLabel = None,
         mode: str = "w",
         encoding: Optional[str] = None,
         compression: CompressionOptions = "infer",

pandas/io/formats/csvs.py

@@ -11,7 +11,12 @@
 import numpy as np
 
 from pandas._libs import writers as libwriters
-from pandas._typing import CompressionOptions, FilePathOrBuffer, Label, StorageOptions
+from pandas._typing import (
+    CompressionOptions,
+    FilePathOrBuffer,
+    IndexLabel,
+    StorageOptions,
+)
 
 from pandas.core.dtypes.generic import (
     ABCDatetimeIndex,
@@ -30,8 +35,6 @@
     infer_compression,
 )
 
-IndexLabel = Optional[Union[bool, str, Sequence[Label]]]
-
 
 class CSVFormatter:
     def __init__(