Upgrade to isort 5.5.3

[hojo0590]

Environment data

• Extension version (available under the Extensions sidebar): unreleased 2020.9

As of now 2020.9 #13924 is going to be released with isort 5.5.2 @karrtikr.
However there is a new version 5.5.3 with a bugfix PyCQA/isort#1488 for a bug that might mangle with a lot of vscode-python user's projects - isort might interfere with lines that are no imports.

Just as a warning...

[karrtikr]

Thanks for the warning, we'll try to push it into next release!