Black is not properly used on `write_cells`

[yasirroni]

Provide a minimally reproducible example

Just run nbdev_export with settings.ini we use black_formatting = False. The __all__ that is produced didn't follow black format.

The problem stems from the fact that the code in config and maker regarding write_cells:

    # write out the file
    with (dest/name).open('w',encoding="utf-8") as f:
        f.write(f"# %% auto 0\n__all__ = {exp_funcs}")
        write_cells(cells, f"# %% {fname.relpath(dest)}", f)
        f.write('\n')

    with self.fname.open('w', encoding="utf-8") as f:
        f.write(_retr_mdoc(cells))
        f.write(f"# AUTOGENERATED! DO NOT EDIT! File to edit: {self.dest2nb}.")
        if last_future > 0: write_cells(cells[:last_future], self.hdr, f)
        if self.parse and not self.solo_nb: f.write(f"\n\n# %% auto 0\n__all__ = {all_str}")
        write_cells(cells[last_future:], self.hdr, f, cell_number=get_config().cell_number, solo_nb=self.solo_nb)
        f.write('\n')

We can see that it always use single quote ' ', even when in settings.ini we use black_formatting = False.

Expected behavior is it follows formatting too.

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Proposal: change formatter runner to run per-file basis, both the .ipynb and .py files.

[yasirroni]

Related to #1544