Provide support for brother-ql-inventree

[FriedrichFroebel]

At the moment, the package requires brother-ql, which is more or less unmaintained. We should evaluate supporting brother-ql-inventree as well: https://pypi.org/project/brother-ql-inventree/

The biggest issue is that they both use the same namespace, thus possibly creating issues or breaking changes:

• The "backend" library is always an extra. This is a breaking change as an extra has to specified in all cases.
• Just brother-ql-inventree is an extra. This creates annoying conflicts as both backend packages would be installed in the same environment, but their code overwriting each other (depending on the installation order). Uninstalling one of them would break both with the current pip logic.

[chrismaster]

Also from the inventree project
Renaming to brother-ql2
https://pypi.org/project/brother-ql2/
and detached fork
https://github.com/matmair/brother_ql2

[FriedrichFroebel]

Are there any informations available on what the differences and maintenance concepts of both of these variants are?

[FriedrichFroebel]

Corresponding issue: matmair/brother_ql-inventree#58