Pin version of setuptools to older one for our build system

Since version 69.3.0 the names of the produced build artifacts
changes, but we prefer to keep the old names for compatibility.

pyproject.toml

@@ -8,7 +8,8 @@
 [build-system]
 requires = [
  # Require versions that support our license files
- 'setuptools >= 42.0.0',
+ # but produce the same names for artifacts as we always had.
+ 'setuptools == 69.2.0',
  'wheel >= 0.32.0',
 ]
 build-backend = 'setuptools.build_meta'