Relax attrs version

[tjader]

Hi, would it be possible to relax the attrs version required and allow newer versions?
It is currently locked to attrs==20.3.0, but from what I can see in the code, the attrs use is fairly limited.

[andynameistaken]

It blocks upgrading mkdocs-jupyter:

mkdocs-markmap 2.4.0 requires attrs==20.3.0, but you have attrs 23.1.0 which is incompatible.

[andynameistaken]

Is it possible to at least update attrs to the last version or this project specifically needs 20.3.0?

[neatc0der]

Update in #55: attrs>=20.3.0