global: reduce pip backtracking for Python 3.10

[GraemeWatt]

Trying to install the HEPData software in a Python 3.10 environment gives an info message:

INFO: This is taking longer than usual. You might need to provide the dependency resolver with stricter constraints to reduce runtime. See https://pip.pypa.io/warnings/backtracking for guidance. If you want to abort this run, press Ctrl + C.

followed eventually by an error message:

ERROR: Exception:
pip._vendor.resolvelib.resolvers.ResolutionTooDeep: 200000

We might need to simplify the dependency resolution, for example, by combining the dependencies from requirements.txt and setup.py, to allow use of Python 3.10. End-of-life for Python 3.9 is not until 5th October 2025, so the move to Python 3.10 is not urgent. See https://pip.pypa.io/warnings/backtracking for more information.