Move to markdown-it-py parser

Implemented in #123

This release implements the move from mistletoe to markdown-it-py, as the underlying markdown parser. The reason for this is are discussed in executablebooks/meta#44 (comment) and executablebooks/meta#47, and the PR #123 discusses in more details the update.

Additional changes:

• Update pydata-sphinx-theme requirement
• Improve testing and move to GitHub Actions CI
• Add tests and fixes for reporter warnings and include directive
• Add documentation of sphinx parser options
• Apply doc fixes suggested by @rossbar in #121
• Add warning for non-consecutive headings

Bump markdown-it-py version

v0.8.0a3

Bump markdown-it-py version

Development Release of Move to markdown-it-py

v0.8.0a1

Update __init__.py

Minor update

v0.7.1

bump version

Update tokens position attribute

v0.7.0

Update tokens to use `mistletoe.Position` rather than old style tuples

Upgrade to mistletoe-ebp~=0.10.0

Introduces Footnotes and improves error reporting, etc (see commit for details)

Bump mistletoe version

v0.6.0a3

bump version

Bump mistletoe version

v0.6.0a2

bump version

Add footnote syntax

v0.6.0a1

bump version

Upgrade to mistletoe-ebp~=0.9.4

v0.5.0

Update API documentation (#111)