fix: make PEP 604 union annotations import-safe on Python 3.9

[mspinola]

Fixes the red CI on main after the volume-reconstruction feature landed (#11).

Failure

src/cotdata/providers/norgate.py:240: in <module>
    def get_symbol_metadata(internal_symbol: str) -> dict | None:
E   TypeError: unsupported operand type(s) for |: 'type' and 'NoneType'

dict | None (PEP 604) is evaluated at import time and only works on Python 3.10+. The CI matrix includes 3.9 (requires-python = ">=3.9"), so collecting test_norgate_provider.py — which imports the norgate provider — fails. cftc._download_year had the same latent Path | None.

Fix

Add from __future__ import annotations to both provider modules, so every annotation is a lazy string and never evaluated at runtime. No behaviour change, and it future-proofs any further annotations in these files.

Verified

• Annotations confirmed stored as strings ('dict | None', 'Path | None') → import-safe on 3.9.
• Full suite: 31 passed locally (3.13).
• Scanned src/ for any other PEP 604 unions — the only two are the ones fixed here.

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