pymupdf: add build-pymupdf.yml for riscv64 wheels

[luhenry]

Adds a riscv64 wheel build for pymupdf 1.28.2.

Shape

build-from-checkout + pypa/cibuildwheel. PyMuPDF's PEP-517 backend (pipcl,
in-tree setup.py) downloads the matching MuPDF source tarball at build time,
builds all of MuPDF (incl. tesseract/leptonica), generates MuPDF's C++ bindings
by parsing its headers with libclang, wraps them with SWIG, and finally compiles
PyMuPDF's own _extra extension — so the wheel job is one long native build.

riscv64-specific overrides

• PYMUPDF_SETUP_LIBCLANG=clang — setup.py's
  get_requires_for_build_wheel() asks for the libclang PyPI wheel, which
  ships no riscv64 build (and its sdist just repackages prebuilt binaries).
  Instead we install the pure-Python clang bindings and let clang.cindex
  dlopen the image's own libclang.so — hence dnf install -y clang-devel in
  CIBW_BEFORE_ALL_LINUX (clang-libs alone has only the versioned soname).
• PYMUPDF_SETUP_SWIG=swig — reuse the manylinux image's swig 4.4.1 instead of
  compiling the swig sdist in-container.
• XCFLAGS/XCXXFLAGS=-D__LITTLE_ENDIAN__ — mupdf's
  scripts/tesseract/endianness.h detects byte order purely from
  arch-specific predefines (__AARCH64EL__, _M_X64, …) and has no riscv64
  case, so it hard-errors with #error "I don't know what architecture this is!". Verified locally in the manylinux riscv64 image.

Matrix

Upstream publishes exactly two Linux wheel shapes: cp310-abi3 and
cp314-cp314t. cp312 is our floor, so the abi3 wheel is built there;
the free-threaded build sets PYMUPDF_SETUP_PY_LIMITED_API=0 because pipcl
asserts Py_LIMITED_API and Py_GIL_DISABLED cannot be combined.

Tests

Upstream's own harness (tests/run_compound.py … pytest tests) with its test
deps, restricted to -i r (the default implementation) — the default rR
reruns the whole suite a second time with PYMUPDF_USE_EXTRA=0.
test_flake8 / test_pylint / test_codespell are ignored: they lint the
source tree rather than exercise the wheel, and track whatever linter version
pip resolves today.