update tox configuration

[p-j-smith]

In CI we are currently pinning tox to version 3 - let's update the tox configuration to be compatible tox v4.

We may also want to define platform specific tox environments for use the the github action, as suggested in the docs for the action:

[tox]
envlist = py{38,39,310}-{linux,macos,windows}

[gh-actions]
python =
    3.8: py38
    3.9: py39
    3.10: py310

[gh-actions:env]
PLATFORM =
    ubuntu-latest: linux
    macos-latest: macos
    windows-latest: windows

[testenv]
deps =
  <common dependency>
  linux: <Linux specific deps>
  macos: <macOS specific deps>
  windows: <Windows specific deps>
...