Binary/source distribution warnings and failures due to now requiring `long_description`

Running twine on the latest version (v0.8.3) gives:

Checking dist/ligo_scald-0.8.3-py3-none-any.whl: FAILED
ERROR    `long_description` has syntax errors in markup and would not be rendered on PyPI.                                                                                                    
         No content rendered from RST source.                                                                                                                                                 
WARNING  `long_description_content_type` missing. defaulting to `text/x-rst`.                                                                                                                 
Checking dist/ligo-scald-0.8.3.tar.gz: PASSED with warnings
WARNING  `long_description_content_type` missing. defaulting to `text/x-rst`.                                                                                                                 
WARNING  `long_description` missing.