update pybind11, confirmed fixes CI Mac

[loriab]

• [x] Switch to new scheme for Python detection available for cmake >=3.15 and recc. by pb11: https://pybind11.readthedocs.io/en/stable/compiling.html?highlight=pybind11_nopython#findpython-mode . Note that all cmake variables go from PYTHON_ to Python_
• [x] Adds flexibility in pb11 detection in that it looks for a prebuilt before invoking FetchContent.
• [x] Updates the FetchContent target from a local 2.4.3 to an upstream 2.6
• [x] Added a flag to allow the pylibint2 tests to pass with pb11 >=2.7.0