Currently Snapcraft only do a `pip install .` operation when the setup.py file is found in the part's source tree[1], which is no longer the case with projects adhering to PEP 518[2] as such projects may have a pyproject.toml file in their source tree instead(e.g. OCRmyPDF[3]). This patch instructs Snapcraft to run the `pip install .` command when pyproject.toml is found in the source tree, according to the pip pull request that implements the support[4] and the Ubuntu package search results[5] this should be supported since core20. [1]: [Line 143 of snapcraft/python.py at 0e2cf91 · snapcore/snapcraft · GitHub](https://github.com/snapcore/snapcraft/blob/0e2cf91/snapcraft_legacy/plugins/v2/python.py#L143) [2]: [PEP 518 – Specifying Minimum Build System Requirements for Python Projects | peps.python.org](https://peps.python.org/pep-0518/) [3]: [Drop setup.py - use pyproject.toml exclusively · ocrmypdf/OCRmyPDF@16fc520](https://github.com/ocrmypdf/OCRmyPDF/commit/16fc520) [4]: [Install build dependencies as specified in PEP 518 by takluyver · Pull Request #4144 · pypa/pip](https://github.com/pypa/pip/pull/4144) [5]: [Ubuntu – Package Search Results -- python3-pip](https://packages.ubuntu.com/search?lang=en&suite=all&exact=1&searchon=names&keywords=python3-pip) LP: [#2008237](https://bugs.launchpad.net/snapcraft/+bug/2008237) - [x] Have you followed the [guidelines for contributing](https://github.com/snapcore/snapcraft/blob/master/CONTRIBUTING.md)? - [x] Have you signed the [CLA](http://www.ubuntu.com/legal/contributors/)? - [ ] Have you successfully run `make lint`? ([nope](https://github.com/snapcore/snapcraft/pull/4087#issuecomment-1493070442), but the CI tests seems to pass?) - [ ] Have you successfully run `pytest tests/unit`? ([nope](https://github.com/snapcore/snapcraft/pull/4087#issuecomment-1493072598), but the CI tests seems to pass?) ----- Note that I don't really familiar with Python, and with software testing in general. Any help to fulfill the merge criteria is appreciated. (CRAFT-2119)