Refs #29942 -- Avoided Python 3.9+ syntax in docs build.

[smithdc1]

Djangoproject.com uses Python 3.8 and therefore the docs build process needs to be compatible with this version.

Dict union operator was added in Python 3.9.

Trac ticket number

Ticket-29942

Branch description

Provide a concise overview of the issue or rationale behind the proposed changes.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.

[smithdc1]

Hi @nessita just to check. Do you have this one on your list before the feature freeze? I'd consider this a release blocker.

I'm happy to help manage this through trac. Should I create a new ticket for it?

[nessita]

Hi @nessita just to check. Do you have this one on your list before the feature freeze? I'd consider this a release blocker.

I'm happy to help manage this through trac. Should I create a new ticket for it?

I'm about to review right now, thanks!

[nessita]

Tested in Python 3.8, docs are built without errors now. Thank you @smithdc1!