Clarify supported Python versions
The package metadata lists support for Python 3.6 to 3.9 here.
However, it also specifies support for Python >=3.6 here.
Is 3.10 still unsupported? If so, we should add an upper limit in that second reference. Otherwise, we should add 3.10 (and I suspect not 3.11) to the first list, and add the appropriate upper limit to the second reference.