fix: pin astroid to 2.4.2

[metrizable]

Issue #, if available:

Description of changes:

a recent update to a transitive dep rolled in that caused breakages: https://pypi.org/project/astroid/#history

changes include:

• pin astroid to 2.4.2 for the time being

Testing done:

❯ tox -e pylint
...
pylint installdeps: pylint, astroid==2.4.2
pylint installed: astroid==2.4.2,isort==5.7.0,lazy-object-proxy==1.4.3,mccabe==0.6.1,pylint==2.6.0,six==1.15.0,toml==0.10.2,wrapt==1.12.1
pylint run-test-pre: PYTHONHASHSEED='231695462'
pylint run-test: commands[0] | python -m pylint --rcfile=.pylintrc -j 0 src/sagemaker

--------------------------------------------------------------------
Your code has been rated at 10.00/10 (previous run: 10.00/10, +0.00)

______________________________________________ summary ______________________________________________
  pylint: commands succeeded
  congratulations :)
tox -e pylint  84.45s user 1.92s system 309% cpu 27.884 total

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