Add setup.py file for build

setup.py

@@ -0,0 +1,25 @@
+import os
+import sys
+
+from setuptools import Extension, setup
+
+NO_EXTENSIONS = (
+    bool(os.environ.get("FROZENLIST_NO_EXTENSIONS"))
+    or sys.implementation.name != "cpython"
+)
+
+if NO_EXTENSIONS:
+    print("*********************")
+    print("* Pure Python build *")
+    print("*********************")
+    ext_modules = None
+else:
+    print("*********************")
+    print("* Accelerated build *")
+    print("*********************")
+    ext_modules = [Extension("frozenlist._frozenlist", ["frozenlist/_frozenlist.c"])]
+
+
+setup(
+    ext_modules=ext_modules,
+)