Cmarkgfm

The output generated by `cmarkgfm.github_flavored_markdown_to_html` for code blocks has changed to be in line with that of github.com.
This change **may break existing css rules** as code blocks were generated differently for `cmarkgfm<0.6.0`.

`diff
- <pre><code class="language-python">print('hello')
+ <pre lang="python"><code>print('hello'
</code></pre>
`

Thank you nschloe for this contribution.

- Add support for Linux aarch64
- Update to `cibuildwheel==v1.10.0`

Thank you odidev for this contribution

- [X] Enable `tasklist` extension

Thank you Andrew-Chen-Wang

- Wheels are now generated using [cibuildwheel](https://github.com/joerick/cibuildwheel) with GitHub Actions
- Add support for Python 3.9
- Document deprecation of Python 2.7 (no longer supported on Windows)

This release has no functional changes compared to 0.5.0.

Thank you fruch for the help with cibuildwheel and thank you theacodes for on-boarding me as a maintainer.