Py3.10: doc build requires sphinx>=4.2

**Description**
In a Python 3.10 env, trying to build docs terminates in an error: ``ImportError: cannot import name 'Union' from 'types' ``

This is a known sphinx error that was fixed in sphinx==4.2: https://github.com/sphinx-doc/sphinx/issues/9562

Currently our requirements file limits the version of sphinx: https://github.com/dwavesystems/dimod/blob/c7b045c1d92db3eb4a9b1c9b575d4556c16390cd/docs/requirements.txt#L2

At least for Py3.10.2, C++ docs are generated fine with sphinx==4.4.

I think we can change the requirement on sphinx version now (py 3.10 --> sphinx>=4.2 etc).

**Environment**
- OS: [e.g., Win10
- Python version: 3.10.2