Simple typing fixes for tasks.py

[gycsaba96]

The main changes are as follows:

• use the _Element class instead of the Element factory for typing
• expect None values often used by lxml
• rename the title setter to set_title
• fix default value for duplicate_cb

This fixes every typing error mypy indicates except for the signature errors of some overrides. (I want to investigate the saved searches before looking into those.)

[diegogangl]

Merged, thanks!