Fix usage with radicale >= 3.5.1

[kraftnix]

Radicale changed acquire_lock in 3.5.1 to take a *args, **kwargs.

If using etesync-dav with a radicale >= 3.5.1, then writes can no longer be performed due to an error as described in #349.

Users of NixOS have the packaged etesync-dav using radicale 3.5.5 (tested working).

This PR would close #349, and should be backwards compatible since we don't use any arguments from *args or **kwargs.