Add compatibility with Pandoc 2.18

It would be nice if you could make this work with Pandoc 2.18 (see [releases](https://pandoc.org/releases.html)).

Currently, at least `conda` / `mamba` complain:

```
Encountered problems while solving:
  - package pantable-0.14.2-pyhd8ed1ab_0 has constraint pandoc >=2.11.2,<2.17 conflicting with pandoc-2.18-h694c41f_0
```

Thanks for your effort!