Make pybind11 an optional build dependency

pybind11 is only used for pybind11.setup_helpers.ParallelCompile, which
parallelises the compilation of the C extensions; it provides no runtime
bindings. Wrap its import and use in try/except ImportError and drop it
from build-system.requires, so the wheel builds without pybind11 (compiling
serially) while still using it for parallel builds when it is installed.

Keep it discoverable via a PEP 735 "build" dependency group, so anyone who
wants parallel source builds can install it with e.g. "pip install --group
build". This lets distributions that do not want the extra build dependency
(e.g. OpenWrt) build Pillow without pulling in pybind11.

Author: commodo

pyproject.toml

@@ -1,13 +1,18 @@
 [build-system]
 build-backend = "backend"
 requires = [
-  "pybind11",
   "setuptools>=77",
 ]
 backend-path = [
   "_custom_build",
 ]
 
+[dependency-groups]
+# Optional; enables parallel C extension compilation (see setup.py).
+build = [
+  "pybind11",
+]
+
 [project]
 name = "pillow"
 description = "Python Imaging Library (fork)"

setup.py

@@ -17,7 +17,6 @@
 import warnings
 from collections.abc import Iterator
 
-from pybind11.setup_helpers import ParallelCompile
 from setuptools import Extension, setup
 from setuptools.command.build_ext import build_ext
 
@@ -33,7 +32,14 @@
     configuration.setdefault(key, []).append(value)
 
 default = int(configuration.get("parallel", ["0"])[-1])
-ParallelCompile("MAX_CONCURRENCY", default).install()
+try:
+    from pybind11.setup_helpers import ParallelCompile
+
+    ParallelCompile("MAX_CONCURRENCY", default).install()
+except ImportError:
+    # pybind11 only provides ParallelCompile, a parallel-build helper; without
+    # it the C extensions are simply compiled serially.
+    pass
 
 
 def get_version() -> str: