Update legacy editable install for torch_em

[anwai98]

While doing editable installs using pip, it returns a depreciation warning stating:

DEPRECATION: Legacy editable install of torch_em==0.7.3 from file:///home/nimanwai/torch-em (setup.py develop) is deprecated. pip 25.0 will enforce this behaviour change. A possible replacement is to add a pyproject.toml or enable --use-pep517, and use setuptools >= 64. If the resulting installation is not behaving as expected, try using --config-settings editable_mode=compat. Please consult the setuptools documentation for more information. Discussion can be found at pypa/pip#11457

Hence, adding pyproject.toml in favor of updating the legacy editable installation of the package from source.