Move to a PEP 517 pyproject.toml build, drop setup.py

[IvanaGyro]

Closes #53.

The only thing setup.py still needed dynamic logic for was appending unicodedata2/pypy_ctype.c to the source list on PyPy. That file existed solely to supply _Py_ctype_toupper[256] for Py_TOUPPER(), which CPython exports from libpython but PyPy's cpyext headers do not define.

Replace it with capability detection in _unicodedata2_compat.h. That header is included after Python.h, which is what makes "#ifdef Py_TOUPPER" meaningful: CPython defines the macro via cpython/pyctype.h (pulled in by Python.h on every non-limited-API build, 3.8 through main), PyPy does not. When it is missing we define UNICODEDATA2_TOUPPER() over a private, static 256-byte table with the same mapping. Detecting the macro rather than _Py_ctype_toupper matters: the latter is a linker symbol the preprocessor cannot see. The table is deliberately not named _Py_ctype_toupper, so nothing is added to CPython's reserved namespace, and it stays static so it is not exported from the extension.

Both branches of the macro apply Py_CHARMASK() to their argument, as Py_TOUPPER() always did, so the call sites no longer mask it themselves. The mask is idempotent and the compiler already folded the duplicate: the .text section is byte identical either way, on both interpreters.

Uppercasing stays table-based on both implementations, so name lookup performance is unchanged; on PyPy the table is now a same-translation-unit static const rather than an extern from a separate object file.

With no implementation-specific source, the extension can be declared statically via setuptools' ext-modules table, and setup.py can go. The build now needs setuptools >= 77.0.1 (declarative ext-modules landed in 74.1, PEP 639 license metadata in 77; 77.0.0 was never published).

Also:

• long_description keeps its rendered form: setup.py injected a "Changelog" setext heading between README.md and CHANGELOG.md, so CHANGELOG.md now carries that heading itself and the two files are concatenated via tool.setuptools.dynamic.
• deploy job builds the sdist with "python -m build --sdist".
• tox envlist points at PyPy versions that still exist.
• CIBW_ENABLE=pypy restores PyPy wheels: cibuildwheel 3.0 moved PyPy into an opt-in enable group, so 17.0.0 and 17.0.1 shipped none, where 16.0.0 and earlier shipped 11-12.