Pin astroid to fix linter issue on travis (#3816)
Pin astroid to fix linter issue on travis (#3816)

* Pin astroid to fix linter issues

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safwanrahman/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org