Remove README.rst from MANIFEST.in
Remove README.rst from MANIFEST.in

``MANIFEST.in`` is referencing non-existing file ``README.rst``.

```
reading manifest template 'MANIFEST.in'
warning: no files found matching 'README.rst'
```

Signed-off-by: Christian Heimes <2314b2e3a4a1f7db165be2aafbf1efd78f28cc97@python.org>

PyO3/setuptools-rust,PyO3/setuptools-rust