Handle cases with pyproject.toml but no setup.py

I've been kicking the tires on COPR's automatic pypi feature, which uses this project to generate spec files. When trying it out with [mdformat](https://github.com/executablebooks/mdformat) I get this error:

```
Running: which pyp2rpm

cmd: ['which', 'pyp2rpm']
cwd: .
rc: 0
stdout: /usr/bin/pyp2rpm
stderr:

Running: pyp2rpm mdformat -t fedora --srpm -d /var/lib/copr-rpmbuild/results -b 3

cmd: ['pyp2rpm', 'mdformat', '-t', 'fedora', '--srpm', '-d', '/var/lib/copr-rpmbuild/results', '-b', '3']
cwd: .
rc: 3
stdout: INFO Pyp2rpm initialized.
INFO Using /var/lib/copr-rpmbuild/results as directory to save source.
INFO Downloaded package from PyPI: /var/lib/copr-rpmbuild/results/mdformat-0.7.11.tar.gz.
INFO Getting metadata from setup.py using SetupPyMetadataExtractor.
stderr: setup.py not found, maybe /var/lib/copr-rpmbuild/results/mdformat-0.7.11.tar.gz is not proper source archive.

Copr build error: setup.py not found, maybe /var/lib/copr-rpmbuild/results/mdformat-0.7.11.tar.gz is not proper source archive.
```

I believe the issue is that somewhere around [PEP 621](https://www.python.org/dev/peps/pep-0621/) it became plausible for a package to have a pyproject.toml but not a setup.py. This is certainly the case with mdformat.

This may be a bigger issue - support pyproject.toml at *all* - but sticking to the scope of the bug report for now.