fix(django): Fix 404 Handler handler being labeled as "generic ASGI request"

[BeryJu]

[github-actions[bot]]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[antonpirker]

It seems that we changed a lot since this PR was submitted originally. @BeryJu I know it is a lot to ask, but could you move your check for the 404 into the new _set_transaction_name_and_source? (you probably have to rebase your branch on master or merge master into your branch