Update isort version in pre-commit config
Previously, a mismatch between the isort versions specified by
pre-commit and by the files used on CI made it impossible to make a
commit which passed both pre-commit and CI lints.

See #36426 . This is not a complete solution as the flake8 version differences are intentional and hopefully temporary (see https://github.com/pandas-dev/pandas/pull/36412#issuecomment-694072443 ), but it should resolve the current breakage.