Pytest-sphinx

- Forgot to bump minimum required pytest version in pytest-sphinx 0.6.2 64

- Adapt to incompatible `import_path` API change in pytest 8.1.1. Pytest 8.1.1 is
required by pytest-sphinx now.

- Require pytest >= 8.0 59

- Drop support for python 3.7 59

- Mention in the README that the doctest directive is supported.
- internal: Improve CI setup, from [Tony
Narlock](https://www.git-pull.com).
- internal: Switch to a src/ structure, from [Tony
Narlock](https://www.git-pull.com).
- internal: Switch from setup.py to a PEP-621 pyproject.toml, from
[Tony Narlock](https://www.git-pull.com).

- Drop python2.7 (Fixes 14)
- Drop python3.5 and python3.6
- Require pytest>=7.0