Add an empty [tool.setuptools_scm] section to pyproject.toml

[musicinmybrain]

See https://setuptools-scm.readthedocs.io/en/latest/usage/. This section is expected to be present, but it may be empty if no non-default settings are required.

This addition does not change the contents of the binary wheels.

Fixes:

$ python3 -m build
[…]
WARNING setuptools_scm.pyproject_reading toml section missing 'pyproject.toml does not contain a tool.setuptools_scm section'
Traceback (most recent call last):
  File "/tmp/build-env-sgdlgtss/lib64/python3.13/site-packages/setuptools_scm/_integration/pyproject_reading.py", line 36, in read_pyproject
    section = defn.get("tool", {})[tool_name]
              ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^
KeyError: 'setuptools_scm'

[musicinmybrain]

Hmm, this fails on ubuntu-latest (only):

  =================================== FAILURES ===================================
  ____________________________ test_uharfbuzz_version ____________________________
  
      def test_uharfbuzz_version():
          v = hb.__version__
          assert isinstance(v, str)
  >       assert "unknown" not in v
  E       AssertionError: assert 'unknown' not in '0.0.0+unknown'
  E         
  E         'unknown' is contained here:
  E           0.0.0+unknown
  
  /project/tests/test_uharfbuzz.py:1607: AssertionError

I’m not quite sure at a glance what it would take to make this work on all supported platforms, so – since this “only” fixes a warning – I’ll close this PR for now.