cleanup: import NoReturn properly + remove unused argparse + fix f-string placeholder

[fadai216]

Three small lints flagged by pyflakes scripts/:

1. kya_lib.py: import NoReturn properly

die() was annotated as -> "NoReturn" (string forward-reference) with a # type: ignore[name-defined] to paper over the missing import. The clean fix is to just add NoReturn to the existing typing import:

-from typing import Any, Optional
+from typing import Any, NoReturn, Optional

-def die(msg: str, code: int = 1) -> "NoReturn":  # type: ignore[name-defined]
+def die(msg: str, code: int = 1) -> NoReturn:

Now mypy / pyright / pyflakes all resolve the name correctly instead of silently trusting the type: ignore.

2. sign-kyc.py:115: f-string without placeholders (F541)

-    print(f"Open this URL in any browser to complete Didit:", flush=True)
+    print("Open this URL in any browser to complete Didit:", flush=True)

Semantically identical, avoids the f"" tripwire that makes future readers look for interpolation that isn't there.

3. sign.py:24: unused import argparse

Script uses base_parser from kya_lib (a pre-configured ArgumentParser), never touches argparse directly. Removed.

Verification

• All 3 files pass python3 -m py_compile
• scripts/test_kya_lib.py still passes 22/22 tests
• pyflakes scripts/ output clean for the 3 flagged items