Fix builds with Cython 3

This is a *de minimis* fix for building with Cython 3. Recent Cython<3
releases provided `Cython.Distutils.build_ext` as an alias to
`Cython.Distutils.old_build_ext.old_build_ext`; Cython 3 drops this
alias and instead uses a wholly new `Cython.Distutils.build_ext` that
does not provide the `cython_sources` function used in `setup.py`.

Explicitly importing `old_build_ext` preserves the existing behavior for
recent Cython<3 and uses the correct behavior for Cython 3. Should the
import fail (*e.g.*, because the version of Cython available predates
the availability of `old_build_ext`), the import falls back to just
`Cython.Distutils.build_ext`.

Signed-off-by: Andrew J. Hesford <ajh@sideband.org>

setup.py

@@ -82,7 +82,11 @@
     with_cython = True
 try:
     from Cython.Distutils.extension import Extension as _Extension
-    from Cython.Distutils import build_ext as _build_ext
+    try:
+        from Cython.Distutils.old_build_ext import old_build_ext as _build_ext
+    except ImportError:
+        from Cython.Distutils import build_ext as _build_ext
+
     with_cython = True
 except ImportError:
     if with_cython: