[AIRFLOW-6063] Remove astroid dependency

Note this dependency was only meant to temporarily fix an issue with pylint and astroid.
See pylint-dev/pylint#3123 for more information.

setup.py

@@ -277,7 +277,6 @@ def write_version(filename: str = os.path.join(*["airflow", "git_version"])):
 # DEPENDENCIES_EPOCH_NUMBER in the Dockerfile
 ############################################################################################################
 devel = [
-    'astroid~=2.2.5',  # to be removed after pylint solves this: https://github.com/PyCQA/pylint/issues/3123
     'beautifulsoup4~=4.7.1',
     'click==6.7',
     'contextdecorator;python_version<"3.4"',