Remove support for old CKAN versions and Python 2

[seitenbau-govdata]

As announced in https://github.com/ckan/ckanext-dcat/pull/241 we cleaned up the extension of code from CKAN versions lower than 2.9 and Python2.

• Removed Python2 compatibility (six and py2 ifelses)
• Removed checks and ifelses for CKAN versions < 2.9
• Removed pylons support
• Removed paster commands
• Cleaned up github actions

[seitenbau-govdata]

The only tests that are currently failing will be fixed with https://github.com/ckan/ckanext-dcat/pull/241.

[seitenbau-govdata]

@amercader and @pdelboca The pull request is now ready for review. Many thanks in advance! 😃

[amercader]

Thanks @seitenbau-govdata ! Can you fix the conflicts so we can merge?

[seitenbau-govdata]

Thanks @seitenbau-govdata ! Can you fix the conflicts so we can merge?

@amercader Done