Backport fixes for REST API version 6.0

The current dev branch as well recent releases focus on version 7.0 & 7.1 of the Azure DevOps REST API ,as mentioned in the documentation of the latter.

However, it would be interesting to backport fixes on top of older versions of the library addressing other REST API versions, matching other versions of the Azure DevOps (Server) product.

For instance, the current v6.0.0b4, the latest to support the 6.0 REST API version matchin Azure DevOps Server 2020.1, does not include fixex to the python package generating warning at the setuptools level when the package is being used.
Cf. this commit.

All fixes being meta or interesting for multiple versions of the REST API would be interesting to get for all the supported product.
Azure DevOps Server 2020, for instance, is supported until 2025 (and potentially 2030).
It is incoherent support for this library moves away from otherwise supported products.

Review the v6.0.0b4 package and the referenced commit that fixed the setuptools warning, then compare fixes on the current dev branch with the supported REST API versions. Define which changes apply to Azure DevOps Server 2020 and other supported versions; done requires an agreed backport scope and corresponding versioned releases.