use != instead of 'is not' with literals

[xloem]

jminardi#76

fix jminardi#72

off-topic: In case you don't find the time to merge pull requests, I suggest to create an "organization" (e.g. called "mecode") at github and copy this repository so it is available at github.com/mecode/mecode and add some contributors as members: https://docs.github.com/en/organizations/collaborating-with-groups-in-organizations/creating-a-new-organization-from-scratch

I am willing to get involved, although I am not a professional developer.