Make the setup.py script importable to enable multiprocessing builds that need...

Make the setup.py script importable to enable multiprocessing builds that need to "spawn" processes (usually on Windows).

setup.py

@@ -188,15 +188,10 @@ try:
 except ValueError:
     compile_cython_itself = True
 
-if compile_cython_itself and (is_cpython or cython_compile_more):
-    compile_cython_modules(cython_profile, cython_compile_more, cython_with_refnanny)
-
 setup_args.update(setuptools_extra_args)
 
-from Cython import __version__ as version
 
-
-def dev_status():
+def dev_status(version):
     if 'b' in version or 'c' in version:
         # 1b1, 1beta1, 2rc1, ...
         return 'Development Status :: 4 - Beta'
@@ -224,7 +219,13 @@ packages = [
     'pyximport',
 ]
 
-setup(
+
+def run_build():
+    if compile_cython_itself and (is_cpython or cython_compile_more):
+        compile_cython_modules(cython_profile, cython_compile_more, cython_with_refnanny)
+
+    from Cython import __version__ as version
+    setup(
         name='Cython',
         version=version,
         url='https://cython.org/',
@@ -257,7 +258,7 @@ setup(
         """),
         license='Apache',
         classifiers=[
-        dev_status(),
+            dev_status(version),
             "Intended Audience :: Developers",
             "License :: OSI Approved :: Apache Software License",
             "Operating System :: OS Independent",
@@ -283,4 +284,8 @@ setup(
         packages=packages,
         py_modules=["cython"],
         **setup_args
-)
+    )
+
+
+if __name__ == '__main__':
+    run_build()