Panflute

Same as `2.1.0` but fixed a dependency (wheels) required to publish to pypi

- autofilter.py: add pandoc 2.12+ support to `get_filter_dirs`
- add `pandoc_path` argument to `run_pandoc()`: `run_pandoc(text='', args=None, pandoc_path=which('pandoc')`
- fix a few docstrings

- Fix 166 ("Regression: panflute.elements.builtin2meta no longer supports subclasses of builtins")
- Improve testing ( 170, 169, 164)

- Fix bug with `replace_keyword()` ; see 104

- Add support for pandoc 2.11.1 (pandoc-types 1.22), which introduced a new Underline element and expanded the capabilities of the Table element.
- Remove support for previous versions of pandoc. Users of previous versions of pandoc should install pre-2.0 versions of panflute
- Fix/add tests
- Add the `pandoc_version` and `pandoc_reader_options` properties of the `Doc()` element, which expose environment variables set by pandoc

- Fix 131: tweak error messages of panflute.dump
- Merge 130: allow slice assignment to element items (via ListContainer)