fix: migrate project.license to SPDX expression (#260)

[patchwright]

Problem

Fixes #260. Building with setuptools >=77 emits two deprecation warnings: project.license as a TOML table, and the License :: OSI Approved :: MIT License trove classifier. Both point at the PEP 639 migration. Confirmed on main HEAD 1d872d3.

Root cause

pyproject.toml uses the PEP 621 table form license = { text = "MIT" } and also lists the License :: classifier. setuptools 77.0.0 (PEP 639) deprecates both in favour of an SPDX string expression.

Fix

• license = { text = "MIT" } -> license = "MIT" (SPDX expression)
• drop the License :: OSI Approved :: MIT License classifier
• bump build-system setuptools>=61 -> >=77 (the SPDX string form needs >=77; the warning itself notes "available on setuptools>=77.0.0")

Metadata only; no runtime change.

How to test

Before:

$ python -m build
SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated ...
SetuptoolsDeprecationWarning: License classifiers are deprecated.
  License :: OSI Approved :: MIT License

After:

$ python -m build
adding license file 'LICENCE'      # no license/classifier deprecation warnings

Full suite: 47 passed.

Backward compatibility

Build floor moves to setuptools >=77 (compatible with requires-python >= 3.9; setuptools 77 supports 3.9+). Runtime unchanged. pip build isolation already resolves >=77.

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Assisted-by: Claude (code generation, reviewed and tested locally)