PyPI sdist is missing files

```py
Traceback (most recent call last):
  File "setup.py", line 12, in <module>
    with open(path.join(here, "requirements.txt"), encoding="utf-8") as f:
FileNotFoundError: [Errno 2] No such file or directory: '/tmp/metaextract_9ojlsku5/pathmagic-0.3.14/requirements.txt'
```

This is often fixed by creating a `MANIFEST.in`