Okay, I just committed a "fix" that will make the opf parser more robust to missing end quotes on attribute values. The repair is not perfect but loses no text.
The non-well formed line:
Code:
<dc:contributor opf:role="art" opf:file-as="Rockwell, Norman>Norman Rockwell</dc:contributor>
will get repaired to the following which should be easy to manually fix without information loss:
Code:
<dc:contributor opf:role="art" opf:file-as="Rockwell, Norman&gt;Norman Rockwell></dc:contributor>
And Sigil will continue to function properly with no Embedded Python Error.
Thank you for your bug report and for your sample code to test things with.