Ignore common non-SPDX licenses

This is meant to catch licenses that are supposed to be SPDX but
aren't, not licenses that *aren't* supposed to be SPDX. We have so
many free-text license descriptions like "Copyrighted" and "Other"
that I'm sick of seeing warnings for them!

CHANGELOG.md

@@ -14,6 +14,7 @@ because it is deprecated and outdated
 - Require Python 3.9+
 - Don't run `fix.separators()` on title fields
 - Don't run whitespace or newline fixes on abstract fields
+- Ignore some common non-SPDX licenses
 
 ### Updated
 - Python dependencies