Installing was broken since moving to pyproject.toml, which we didn't notice because of leftover files in build/. Fix this by using the convention of having the source files in src/ and adjusting pyproject.toml accordingly. Fixes gh-86. 🤞
This reverts commit a3c1eee8f31349edcfb1e36920763bcecceb1129, reversing changes made to dc76213ffc1fbabc2c45f0e52ced55449bdf2e83.