Remove call-stack inspection from DateTime.__add__

[ariebovenberg]

While investigating #818, I found a big contribution to the slowdown to be a stack walk in DateTime.__add__:

caller = traceback.extract_stack(limit=2)[0].name
if caller == "astimezone":
    return super().__add__(other)

It turns out this exists because ZoneInfo.from_utc() expects arithmetic with standard library semantics when called as part of astimezone. Using the stack walk, DateTime detects this situation and reverts to stdlib semantics. This approach has a high cost (it's slow, and has side effects like #820 ), but there is a relatively cheap alternative: convert DateTime to a datetime before calling astimezone(). The same solution is proposed in #1006 (it doesn't remove the stack walk though).

With this approach, + becomes ±5x faster. in_tz/astimezone become ±10x faster.

There's a theoretical behaviour change: calling + from a function that happens to be called astimezone() will get a different result.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code. n/a

AI disclosure: I used Claude Code to code this up. I have reviewed the code thoroughly.

edit: clarity relationto #1006

[codspeed]

Merging this PR will not alter performance

✅ 1 untouched benchmark

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_{Comparing ariebovenberg:remove-stack-walk (e7e8ea4) with master (b8ebebe)}

[Secrus]

Thank you for that improvement!