[wishlist]: Read calibration data from pypeit instead of downloading it

[Hellseher]

Hi,

While preparing Guix Astro Update 2025/11 and refreshing python-specreduce to 1.7.0 I've noticed
that it tried to download calibration data from GitHub for one test:

    def load_pypeit_calibration_lines(
            lamps: Sequence | None = None,
            wave_air: bool = False,
            cache: bool | Literal['update'] = True,
            show_progress: bool = False
    ) -> QTable | None:
        """
        Load reference calibration lines from ``pypeit`` linelists. The ``pypeit`` linelists are
        well-curated and have been tested across a wide range of spectrographs. The available
        linelists are defined by ``PYPEIT_CALIBRATION_LINELISTS``.

        Parameters
        ----------
        lamps : Lamp string, comma-separated list of lamps, or sequence of lamps to include in
            output reference linelist. The parlance of "lamp" is retained here for consistency
            with its use in ``pypeit`` and elsewhere. In several of the supported cases the
            "lamp" is the sky itself (e.g. OH lines in the near-IR).
            The available lamps are defined by ``PYPEIT_CALIBRATION_LINELISTS``.

        wave_air : If True, convert the vacuum wavelengths used by ``pypeit`` to air wavelengths.

        cache : Toggle caching of downloaded data

        show_progress : Show download progress bar

        Returns
        -------
        linelist:
            Table containing the combined calibration line list. ``pypeit`` linelists have the
            following columns:
            * ``ion``: Ion or molecule generating the line.
            * ``wavelength``: Vacuum wavelength of the line in Angstroms.
            * ``NIST``: Flag denoting if NIST is the ultimate reference for the line's wavelength.
            * ``Instr``: ``pypeit``-specific instrument flag.
            * ``amplitude``: Amplitude of the line. Beware, not consistent between lists.
            * ``Source``: Source of the line information.
        """
        if lamps is None:
            return None

        if not isinstance(lamps, Sequence):
            raise ValueError(f"Invalid calibration lamps specification: {lamps}")

        if isinstance(lamps, str):
            if ',' in lamps:
                lamps = [lamp.strip() for lamp in lamps.split(',')]
            else:
                lamps = [lamps]

        linelists = []
        for lamp in lamps:
            if lamp in PYPEIT_CALIBRATION_LINELISTS:
                data_url = f"{PYPEIT_DATA_URL}/arc_lines/lists/{lamp}_lines.dat"
                try:
                    data_path = download_file(data_url, cache=cache,
                                              show_progress=show_progress,
                                              pkgname='specreduce')
                    linelists.append(Table.read(data_path, format='ascii.fixed_width', comment='#'))
                except URLError as e:
>                   warnings.warn(f"Downloading of {data_url} failed: {e}", AstropyUserWarning)
E                   astropy.utils.exceptions.AstropyUserWarning: Downloading of https://raw.githubusercontent.com/pypeit/pypeit/release/pypeit/data//arc_lines/lists/ArI_lines.dat failed: <urlopen error [Errno -3] Temporary failure in name resolution>

specreduce/calibration_data.py:167: AstropyUserWarning
=========================== short test summary info ============================
FAILED specreduce/tests/test_wavecal1d.py::test_init_line_list - astropy.utils.exceptions.AstropyUserWarning: Downloading of https://raw.githubusercontent.com/pypeit/pypeit/release/pypeit/data//arc_lines/lists/ArI_lines.dat failed: <urlopen error [Errno -3] Temporary failure in name resolution>
================= 1 failed, 104 passed, 24 deselected in 6.35s =================

However pypeit provides data in PyPI package, see https://github.com/pypeit/PypeIt/tree/1.17.4/pypeit/data.
May it be helpful to have some environment variable to check for calibration data from or try to read it from the pypeit package?

Thanks,
Oleg