Bump pybind11 version, include in Py dependencies, support pathlib.Path

[nilsvu]

Proposed changes

Mostly to support Python's pathlib.Path in bindings. Also these commits make CI testing on Apple Silicon easier.

Upgrade instructions

Pybind11 version 2.7+ is required now. To upgrade, run pip install -r support/Python/requirements.txt in your Python environment.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[knelli2]

Is pybind11 v2.7.0 actually installed in the container? We may need to pip install it