Use new method to avoid pypandoc warning

This fixes the following deprecation warning:

```
setup.py:8: DeprecationWarning: Due to possible ambiguity, 'convert()' is deprecated. Use 'convert_file()'  or 'convert_text()'.
  read_md = lambda f: convert(f, 'rst')
```

Example:
(b
 [0])

Use NotImplementedError rather than NotImplemented