Regdown

- Replace the use of deprecated markdown.util.etree
- Changed Python 3.7 to 3.10
- Pin markdown under 3.5

- Include attr_list, allowing regdown users to set html attributes

- Replace Travis with GitHub Actions and Replace Travis Badge with Github Action Badge
- Remove unnecessary dependency on sha3 package

- Update for Markdown 3.2, drop 3.0 support 2
- Changed em_strong to em_strong2
- Dropped support for Python 2.7
- Changed Python 3.7 to 3.8

- Fix the HTML em dash entity [1](https://github.com/cfpb/regdown/pull/1)

- Initial release!