Pin the build backend so sdist filenames stay PEP 625 compliant

[niconoe]

Building 0.17.0 produced python-dwca-reader-0.17.0.tar.gz, which PyPI rejects:

400 Filename 'python-dwca-reader-0.17.0.tar.gz' is invalid,
    should be 'python_dwca_reader-0.17.0.tar.gz'.

Cause

PEP 625 requires sdist filenames to use the normalised project name. setuptools started
emitting that form in 69.3, but python setup.py sdist builds with whatever setuptools
happens to be first on PATH. On the release machine that was 68.0.0, which predates the
change, so the sdist got the old dash form. (Wheels were unaffected - they have always used
the normalised name.)

Fix

Declaring the build backend means python -m build resolves its own requirements in an
isolated environment rather than inheriting the caller's setuptools.

Verified rather than assumed: built the project from a virtualenv deliberately pinned to
setuptools==68.0.0 - the exact failure condition - and the artifact is still named
python_dwca_reader-0.17.0.tar.gz, because isolation honours the floor declared here.

Checked the change is otherwise inert:

• the sdist's file list is identical to the one built before, apart from pyproject.toml
  itself now being included
• Metadata-Version, Name, Version and Requires-Python are unchanged
• python -m pip install . (what CI runs) still works and the installed package reads
  archives correctly
• the test suite is unchanged at 222 passing

Package metadata deliberately stays in setup.py. This only pins how the build happens, not
what is built - a PEP 621 migration is a separate change and not one to make immediately after
a release.

Also

Sets long_description_content_type="text/x-rst". README.rst is reStructuredText and that is
already the inferred default, so this changes nothing about the rendered page - it just silences
the warning twine prints on every upload. twine check now reports PASSED with no warnings.

Not included

The build/ directory left behind by setup.py shadows the PyPA build module, so
python -m build from a dirty working tree fails with a confusing "No module named build".
Worth knowing when releasing; not worth a code change here.