Fix CI: update Python versions and test runner for modern GitHub Actions

[Copilot]

Python 3.7 and 3.8 are no longer available in GitHub Actions runners (Ubuntu 24.04), causing all CI jobs to fail at the setup-python step. Additionally, nose is unmaintained and incompatible with Python 3.12+.

Workflow & tooling

• Python matrix: [3.7, 3.8] → ["3.11", "3.12"]
• actions/checkout@v2 → v4, actions/setup-python@v2 → v5
• Test runner: nosetests → pytest --cov=ibflex
• requirements-development.txt: replace nose + coverage with pytest + pytest-cov
• setup.cfg: replace [nosetests] config block with [tool:pytest]

Pre-existing mypy errors (would have failed the mypy CI step)

• ibflex/Types.py: removed duplicate field currency in EquitySummaryByReportDateInBase and 10 duplicate fields in SymbolSummary (e.g. ibExecID, openDateTime, tradePrice, etc.)
• ibflex/parser.py: corrected parse_element return type to Optional[Union[FlexElement, Tuple[FlexElement, ...]]]; used cast() to narrow tuple type after None-filtering; added assert known is not None before use inside tolerance branch

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