Add pyproject.toml

[e-nomem]

Admittedly, there are a few more things going on in this PR other than the pyproject.toml:

1. Bumped the minimum python version to 3.9 (I brought this up in Update minimum python to 3.9? #97, but we can just put 3.5 here instead if support for older pythons can't be dropped, might have to fiddle around with the min bounds on setuptools though)
2. Changed the repo layout to a src-layout
   • This is less error-prone in terms of ensuring things don't accidentally end up in the distribution, but makes using an editable install for local development mandatory. An editable install or in-place build was already required previously though so I don't think the developer experience is any worse because of this change.
3. Moved the tests to a dedicated folder in the root
   • They will still be included in the sdist, but not in any wheel or install