Prepare for astroid 2.5.6

Author: Pierre-Sassoulas

ChangeLog

@@ -6,6 +6,13 @@ What's New in astroid 2.6.0?
 ============================
 Release Date: TBA
 
+What's New in astroid 2.5.6?
+============================
+Release Date: 2021-04-25
+
+* Fix retro-compatibility issues with old version of pylint
+ Closes PyCQA/pylint#4402
+
 What's New in astroid 2.5.5?
 ============================
 Release Date: 2021-04-24

astroid/__pkginfo__.py

@@ -28,7 +28,7 @@
 
 from typing import Optional
 
-__version__ = "2.5.5"
+__version__ = "2.5.6"
 # For an official release, use 'alpha_version = False' and 'dev_version = None'
 alpha_version: bool = False # Release will be an alpha version if True (ex: '1.2.3a6')
 dev_version: Optional[int] = None