fix: resolve pylint W1514 (unspecified-encoding) and re-enable it in CI lint

[sbnair]

Contributes to #1007 — completely resolves one of the disabled pylint rules, W1514 unspecified-encoding (13 violations: 12 in qlib/, 1 in an example notebook), and removes it from the disable lists in the Makefile so CI enforces it from now on.

Unlike purely cosmetic rules, W1514 guards against a real bug class: without an explicit encoding, Python's open() uses the platform locale encoding. On Chinese-locale Windows (a large share of Qlib's user base) that is cp936/GBK, so reading UTF-8 YAML configs, JSON reports, or calendar files can raise UnicodeDecodeError or silently corrupt text.

Changes

• Added encoding="utf-8" to every text-mode open() / Path.open() in qlib/ (config loading in qlib/__init__.py, qlib/cli/run.py, qlib/utils/__init__.py, qlib/contrib/rolling/base.py, qlib/rl/contrib/*, TRA's info.json dump, and calendar/instrument files in qlib/data/storage/file_storage.py) and in examples/benchmarks/TRA/Reports.ipynb.
• qlib/utils/file.py::get_io_object(): since it forwards *args, **kwargs to Path.open(), it now defaults to UTF-8 only when the file is opened in text mode without an explicit encoding; binary mode and caller-provided encodings are untouched.
• qlib/data/storage/file_storage.py::_write_calendar(): kept as-is (its mode is always binary "wb"/"ab", and np.savetxt already encodes via its own encoding="utf-8" argument) with an explanatory comment and a targeted pragma, as pylint cannot infer the dynamic mode.
• Makefile: removed W1514 from the pylint (qlib + scripts) and nbqa disable lists.

Verification

• make pylint (with W1514 now enforced): 10.00/10 on both qlib and scripts (pylint 4.0.6).
• nbqa pylint . --enable=W1514: no remaining violations in notebooks.
• black -l 120 --check: all touched files unchanged.
• Runtime-tested get_io_object(): UTF-8 text read of non-ASCII content, explicit encoding= still respected, binary mode (positional and keyword) unaffected.

All changes are behavior-preserving on UTF-8 locales (Linux/macOS defaults) and fix locale-dependent behavior on Windows.

[sbnair]

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