debugpy: add build-debugpy.yml for riscv64 wheels

[luhenry]

Adds a riscv64 wheel build for debugpy 1.8.21.

Why

debugpy publishes per-interpreter manylinux_2_34_x86_64 wheels that carry pydevd's two
Cython speedups — _pydevd_bundle/pydevd_cython*.so and
_pydevd_sys_monitoring/_pydevd_sys_monitoring_cython*.so — plus a py2.py3-none-any
fallback. On riscv64 today pip install debugpy lands on the pure-Python fallback, so the
debugger runs its slow tracer. This builds the compiled wheels.

Shape

Build-from-checkout via cibuildwheel, matrix cp312/cp313/cp314. pydevd ships the
pre-generated .c next to each .pyx, so nothing needs Cython at build time.

Notes:

• REQUIRE_CYTHON_BUILD=1 is load-bearing (CLAUDE.md gotcha 20): setup.py's
  build_py.finalize_options shells out to setup_pydevd_cython.py build_ext and
  swallows the failure, and ext_modules is a truthy empty list, so without it a
  broken compile still produces a correctly-tagged cp3XX-manylinux_riscv64 wheel with no
  .so in it.
• PYDEVD_USE_CYTHON=yes in the test phase makes pydevd_cython_wrapper raise instead of
  silently falling back to the pure-Python tracer, so the whole suite exercises the
  extension; a direct import probe runs ahead of pytest as well.
• CIBW_TEST_SOURCES: tests pytest.ini — tests/ is a package whose conftest.py
  registers tests.pytest_fixtures/tests.pytest_hooks, so it has to sit in the test cwd,
  and pytest.ini carries testpaths/-n8.
• Test requirements are tests/requirements.txt minus gevent (not on our registry and no
  riscv64 wheel on PyPI; its single test is --ignored, and it self-skips on 3.13+ anyway)
  and the build-only vswhere/Cython.

Attach-by-PID and free-threading

The first CI run put both of these on the record, so they are deselections with evidence
rather than guesses.

• attach_pid is skipped below 3.14. pydevd injects its attach shim from a per-arch
  prebuilt library, and add_code_to_python_process.py only accepts
  arm64/amd64/x86/x86_64/i386 — there is no riscv64 build of it upstream, so attach-by-PID
  cannot work. CPython 3.14 attaches through sys.remote_exec() instead
  (debugpy/server/cli.py), which needs no shim. That split is exactly what CI showed: the
  cp314 job passed all 1330 tests, while cp312 and cp313 failed 100 tests each and every
  one of them was an attach_pid parametrisation (Timed out waiting for debug server to connect). So -k "not attach_pid" is applied on cp312/cp313 only, and 3.14 keeps the
  full coverage.
• No cp314t build. debugpy ships no free-threaded wheel on PyPI, its tox.ini envlist
  stops at py314, and its classifiers list no free-threading support. The cp314t job
  matched that: 40 pytest-xdist workers crashed outright, spread across every test module
  (test_threads, test_evaluate, test_args, test_exception, …), not a subset that
  points at one feature. Building it would mean shipping riscv64 a configuration upstream
  supports nowhere.

Upstream builds its released wheels in a private pipeline; the public
azure-pipelines/pipelines.yaml runs the same pytest suite through tox, which is what the
test command here mirrors.