refactor: transition to pyproject.toml

[Dev-iL]

Description

Migrate Flask-AppBuilder's packaging from legacy setup.py to modern pyproject.toml (PEP 621), using the same setuptools backend.

What changes:

• New pyproject.toml with [build-system], [project] (all metadata, 21 dependencies, 4 optional-dependency groups, entry-point), [tool.setuptools] (include-package-data, package-data patterns, dynamic version via attr), [tool.mypy] (global settings + 7 per-module overrides), and [tool.coverage.run].
• Deleted setup.py — replaced entirely by pyproject.toml.
• Deleted MANIFEST.in — replaced by explicit [tool.setuptools.package-data] patterns for static/templates/translations.
• Trimmed setup.cfg — removed all [mypy] and [mypy-*] sections (moved to pyproject.toml). Retained [unittest], [coverage] (nose2 plugin), and [flake8] sections.
• Updated flask_appbuilder/__init__.py — added importlib.metadata.version() as the primary version source with the hardcoded string as fallback, ensuring both flask_appbuilder.__version__ and importlib.metadata.version('Flask-AppBuilder') return consistent values.
• Updated .github/workflows/ci.yml — removed setup.py from the black formatting check.
• Updated tox.ini — removed setup.py from the black check target.
• Updated release.sh — replaced python setup.py clean --all sdist bdist_wheel with python -m build; cleans both ./dist and ./build directories.
• Updated RELEASE.rst — step 8 now uses python -m build && twine upload dist/*.
• Updated requirements/dev.in — added build package as an explicit dev dependency.

Key design decisions:

• requires-python updated from ~=3.7 to >=3.9 (3.7 and 3.8 are EOL; matches CI matrix).
• Python 3.7/3.8 classifiers removed; License classifier removed (superseded by PEP 639 license = "BSD-3-Clause" expression).
• Uses [tool.setuptools.packages.find] include = ["flask_appbuilder*"] instead of exclude = ["tests*"] to prevent unintended discovery of top-level directories (bin/, examples/, docs/).
• Explicit [tool.setuptools.package-data] for static/templates/translations ensures reliable inclusion in isolated builds where VCS access may be unavailable.
• __version__ kept as a simple top-level string assignment for setuptools attr static analysis compatibility, with runtime override from importlib.metadata.

Verified:

• Built wheel file listing is identical before and after migration (243 files).
• All 21 dependency version specifiers are semantically equivalent.
• Editable install (pip install -e .) and regular install (pip install .) both succeed.
• All package metadata (name, author, license, description, URLs, classifiers, entry-points) preserved.

ADDITIONAL INFORMATION

• Has associated issue:
• Is CRUD MVC related.
• Is Auth, RBAC security related.
• Changes the security db schema.
• Introduces new feature
• Removes existing feature

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.56%. Comparing base (eefc219) to head (71a875c).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2444      +/-   ##
==========================================
- Coverage   79.91%   79.56%   -0.36%     
==========================================
  Files          81       81              
  Lines        9183     9250      +67     
==========================================
+ Hits         7339     7360      +21     
- Misses       1844     1890      +46     

Flag	Coverage Δ
python	79.56% <100.00%> (-0.36%)	⬇️

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