build: replace Pool with ThreadPool to avoid pickle issues (#88335)

setup.py

@@ -9,7 +9,7 @@ from distutils.command.build import build as _build
 from distutils.command.sdist import sdist
 from distutils.errors import CompileError
 from distutils.spawn import find_executable
-from multiprocessing import Pool
+from multiprocessing.pool import ThreadPool
 
 from setuptools import find_packages, setup
 from setuptools.command.install_lib import install_lib as _install_lib
@@ -140,7 +140,7 @@ class compile_scss(Command):
             for css_filename in get_css_files():
                 yield ['sassc', '--sourcemap', css_filename.replace('.css', '.scss'), css_filename]
 
-        with Pool() as pool:
+        with ThreadPool() as pool:
             for result in pool.imap_unordered(subprocess_run, get_build_commands()):
                 if not result:
                     raise CompileError('error compiling css')
@@ -189,7 +189,7 @@ class build_icons(Command):
                 ]
             )
 
-        with Pool() as pool:
+        with ThreadPool() as pool:
             for result in pool.imap_unordered(subprocess_run, cmds):
                 if not result:
                     raise CompileError('error building icons')