pybuild and optional dependencies

[PICCA Frederic-Emmanuel <frederic-emmanuel.picca@synchrotron-soleil.fr>]

Hello, I am updating the xraylarch package which contain something like this in the setup.cfg

```
install_requires =
    asteval>=0.9.28
    numpy>=1.20
    scipy>=1.7
    uncertainties>=3.1.4
    lmfit>=1.2.1
    pyshortcuts>=1.9.0
    xraydb>=4.5
    silx>=0.15.2
    matplotlib>=3.5
    sqlalchemy>=2.0
    sqlalchemy_utils
    h5py>=3.2
    hdf5plugin
    pillow>=8.3.2
    numdifftools
    pandas
    packaging
    pip
    pyyaml
    toml
    termcolor
    dill
    imageio
    charset-normalizer
    peakutils
    requests
    scikit-image
    scikit-learn
    psutil
    pymatgen
    mp_api
    pycifrw
    fabio
    pyfai
    numexpr==2.8.4; python_version < '3.9'
    numexpr>=2.8.7; python_version > '3.8'

[options.extras_require]
wxgui =
    wxpython>=4.1
    wxutils>=0.3.0
    wxmplot>=0.9.57

qtgui =
    PyQt5
    pyqtwebengine
    pyqtgraph

epics =
    pyepics>=3.5.0
    psycopg2-binary

jupyter =
    jupyter_core>=5.0
    jupyter_client
    jupyter_server
    notebook
    nbformat
    ipywidgets
    plotly
    py3dmol

doc =
    sphinx
    numpydoc
    sphinxcontrib-bibtex
    sphinxcontrib-argdoc
    pycairo; platform_system=="Windows"

dev =
    pytest
    pytest-cov
    coverage
    build
    pre-commit
    twine

larix =
    %(wxgui)s
    %(jupyter)s

all =
    %(dev)s
    %(doc)s
    %(wxgui)s
    %(qtgui)s
    %(jupyter)s
    %(epics)s
```

When I compile the package, I got the dh_python3 computed runtime dependencies from the install_requires.

Now I would like to build this package but with the larix optional dependencies.

so I added all the dependencies in the Build-Depends, but dh_python3 still produce the previous dependencies.

How can I teach pybuild  that I really want xraylarch[larix] ?

thanks

Frederic