Migrate external_deps prettytable to pypi prettytable (#18661)

[QuantumManiac]

Summary:

Migrate external_deps prettytable to direct pypi dep. Removes tp2 external_deps reference and replaces with fbsource//third-party/pypi/prettytable:prettytable in deps.

Part of the tp2 external_deps migration effort (T261267852).

#buildmore

This diff has been authored with assistance from Claude Code

Differential Revision: D97995123

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18661

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✅ You can merge normally! (2 Unrelated Failures)

As of commit 3cd6f1e with merge base 15b268f ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (matched win rule in flaky-rules.json)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@QuantumManiac has exported this pull request. If you are a Meta employee, you can view the originating Diff in D97995123.

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