switch to sip 6

pyproject.toml

 # Specify sip v5 as the build system for the package.
 [build-system]
-requires = ["sip >=5, <6", "auditwheel"]
+requires = ["sip >=5", "auditwheel"]
 build-backend = "sipbuild.api"
 
 # Specify the PEP 566 metadata for the project.