Modernize the code base

[geigerzaehler]

A couple of modernizations for the code base in separate commits

[wisp3rwind]

Oh, wow, I didn't realize we still depended on six here. Thanks for finally wrapping up the Python 3 transition!

I guess the new assert statements are there to calm the type checker?

Otherwise, looks at a quick glance 👍

Regarding the Python 3.13 failures, cf. beetbox/mediafile#67

[geigerzaehler]

Oh, wow, I didn't realize we still depended on six here.

Yeah, I had always put this off because I thought it would be more effort.

I guess the new assert statements are there to calm the type checker?

Yes