style: refine code style reported by pylint and flake8 (#1007)

[Yash Dharmendra Parmar (Yashparmar1125)]

Description

This PR addresses issue #1007 by resolving code style violations reported by pylint and flake8 across the repository without altering any runtime functionality.

Key Changes

• Resolved all F541 (flake8) and W1309 (pylint) f-string placeholder violations across qlib, scripts, and examples.
• Removed W1309 from the pylint disable list and F541 from the flake8 ignore list in Makefile.
• Fixed missing import pickle in examples/rl_order_execution/scripts/merge_orders.py.
• Cleaned unused imports and unused exception bindings across qlib, scripts/data_collector, and tests.
• Replaced star import with explicit imports in tests/rl/test_saoe_simple.py.
• Configured Black in pyproject.toml and updated Makefile to exclude local virtual environments.

Verification

• flake8: PASSED with zero errors.
• pylint: Rated 10.00/10 on both qlib and scripts.
• black: PASSED (333 files checked, 0 formatting changes).

Fixes #1007

[Yash Dharmendra Parmar (Yashparmar1125)]

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