I want to know if it's possible to edit pages document using Python or other scripting tools. I need to modify the paragraph style of lines containing a date (e.g. 2020/05/02), which can be easily done by a script. But I don't know how to do it in Pages.
There are two approaches you can take.
OSA / AppleScript
Use macOS's Open Scripting Architecture (OSA) to interact with the Pages application.
Decompress, Edit, and Recompress
Alternatively, you can directly edit the Pages document using a custom script.
Pages documents used to be a compressed archive of XML and supporting files. More recent iWork documents (Pages, Numbers and Keynote) use iwa named files which are also documented but compressed versions of the data.
From the excellently documented open source project iWorkFileFormat comes this description:
If this sort of technical scripting is your idea, start by unzipping the file, apply your script to the resulting data files, and recompress.