Add formatting, docs-build and wheel-install checks to CI; drop Python 3.9

[apdavison]

Fixes #124.

Currently, CI runs flake8 (syntax errors and undefined names only) and pytest, always against an editable install. This PR adds three jobs alongside the test matrix:

• format: black --check --diff ., with black pinned to major version 26, so that the new stable style that black introduces each January doesn't make every pull request fail. Updating black then means bumping the pin in a commit that also reformats the code.
• docs: builds the documentation with Sphinx on Python 3.11, as ReadTheDocs does, with -W --keep-going, so that a broken reference or malformed markup fails the pull request rather than surfacing later on ReadTheDocs. The documentation currently builds with no warnings.
• package: builds the wheel, installs it in a clean virtual environment and imports every fairgraph module from it, outside the source tree, so that a packaging error (e.g., a subpackage missing from [tool.setuptools.packages.find]) cannot reach PyPI. This check found the broken fairgraph.fields module removed in Remove deprecated aliases from the renaming of fields to properties #161.

The contributing guide now gives the black and sphinx-build commands, so these checks can be run locally.

A second commit drops support for Python 3.9, which has reached end of life, and adds Python 3.15 to the test matrix. requires-python becomes >=3.10, and black's target version becomes py310 (which does not change the formatting). The latest releases of requests and tabulate already require Python 3.10. The 0.16.0 release notes mention the change.