Upgrade the version to 2.9.1-dev0 following 2.9.0 release

Author: Pierre-Sassoulas

astroid/__pkginfo__.py

@@ -24,5 +24,5 @@
 # Licensed under the LGPL: https://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html
 # For details: https://github.com/PyCQA/astroid/blob/main/LICENSE
 
-__version__ = "2.9.0"
+__version__ = "2.9.1-dev0"
 version = __version__

tbump.toml

@@ -1,7 +1,7 @@
 github_url = "https://github.com/PyCQA/astroid"
 
 [version]
-current = "2.9.0"
+current = "2.9.1-dev0"
 regex = '''
 ^(?P<major>0|[1-9]\d*)
 \.