setup: force use of *.c files if WITHOUT_CYTHON=1 (environment variable)

setup.py

 from setuptools import setup, Extension
+import os
 
 kw = dict(name="zstd._bindings", libraries=["zstd"])
 try:
+    if int(os.getenv("WITHOUT_CYTHON") or 0):
+        raise ImportError
     from Cython.Build import cythonize
 except ImportError:
     ext_modules = [Extension(sources=["zstd/_bindings.c"], **kw)]