Fix the documentation for `delete_duplicates`.
Intersphinx with latest python 3.
Include `delete_duplicates` in the doc page.

Also fix the references.
Fix references to __getitem__.
Note the addition of ignore_underscore in 1.5.5.
Add the documentation for DynamicClassAttributes.
Maintain the intersphinx cache for ever.
Add the `crypto` module documentation.
Update changelog for release 1.5.5.
Merge branch 'develop' into release-1.5.5
Mark the release of 1.5.5.