Log Message:
py-octoprint-filecheck: update to version 2021.2.23

The changes include:

 * #2 Fix native grep not always being in extended regex mode
 * Add a "Prerelease" release channel
 * Code now uses pre-commit, black, prettier, isort and flake8 just like \ 
OctoPrint itself.

In turn, this is required to update py-octoprint to version 1.6.1.