[ty] Add dd-trace-py to flaky.txt (#25391)

Unfortunately this project is now flaky following
https://github.com/astral-sh/ruff/commit/95eec58af266ba0f98bb923cf204f442523e813d.
Adding it to flaky.txt means we don't get spurious/confusing diffs like
https://github.com/astral-sh/ruff/pull/25017#issuecomment-4390844917
anymore, because ecosystem-analyzer will run the project several times
to ensure that newly reported diagnostic diffs are not flaky. The cost
is that ecosystem-analyzer will take a little longer in CI