fixing Cypthon dependency

force setuptools to install Cython if it's not installed

setup.py

@@ -4,39 +4,59 @@ import subprocess
 from distutils.core import setup
 from distutils.extension import Extension
 from distutils.command.build_ext import build_ext
-from Cython.Build import cythonize
+
+CYTHON_VERSION = 'Cython==0.27.2'
 
 class prepare_tinydtls(build_ext):
     def run(self):
         def run_command(args):
             print("Running:", " ".join(args))
             subprocess.check_call(args, cwd=os.path.join(os.path.dirname(__file__), "DTLSSocket","tinydtls"))
-        commands = [
-            ["autoconf"],
-            ["autoheader"],
-            ["./configure", "--without-ecc"],
-            ]
+        commands =  [
+                    ["autoconf"],
+                    ["autoheader"],
+                    ["./configure", "--without-ecc"],
+                    ]
         if not os.path.exists(os.path.join(os.path.dirname(__file__), 'DTLSSocket','tinydtls','dtls.c')):
             run_command(["git", "submodule", "update", "--init"])
         for command in commands:
             run_command(command)
         build_ext.run(self)
 
-cy_build = cythonize([
-      Extension("DTLSSocket.dtls",
-                ["DTLSSocket/dtls.pyx",
-                 "DTLSSocket/tinydtls/dtls.c", "DTLSSocket/tinydtls/crypto.c",
-                 "DTLSSocket/tinydtls/ccm.c", "DTLSSocket/tinydtls/hmac.c", "DTLSSocket/tinydtls/netq.c",
-                 "DTLSSocket/tinydtls/peer.c", "DTLSSocket/tinydtls/dtls_time.c",
-                 "DTLSSocket/tinydtls/session.c", "DTLSSocket/tinydtls/dtls_debug.c",
-                 "DTLSSocket/tinydtls/aes/rijndael.c", "DTLSSocket/tinydtls/sha2/sha2.c"],
-                include_dirs=['DTLSSocket/tinydtls'],
-                define_macros=[('DTLSv12', '1'),
-                               ('WITH_SHA256', '1'),
-                               ('DTLS_CHECK_CONTENTTYPE', '1'),
-                               ('_GNU_SOURCE', '1')],
-                undef_macros = [ "NDEBUG" ],
-                )])
+# force setuptools to install Cython before proceeding
+try:
+    from Cython.Build import cythonize
+except:
+    from setuptools.dist import Distribution
+    Distribution(dict(setup_requires=CYTHON_VERSION))
+
+def get_ext_modules():
+    from Cython.Build import cythonize
+    return cythonize([
+          Extension("DTLSSocket.dtls",
+                    [
+                     "DTLSSocket/dtls.pyx",
+                     "DTLSSocket/tinydtls/ccm.c",
+                     "DTLSSocket/tinydtls/crypto.c",
+                     "DTLSSocket/tinydtls/dtls.c",
+                     "DTLSSocket/tinydtls/dtls_debug.c",
+                     "DTLSSocket/tinydtls/dtls_time.c",
+                     "DTLSSocket/tinydtls/hmac.c",
+                     "DTLSSocket/tinydtls/netq.c",
+                     "DTLSSocket/tinydtls/peer.c",
+                     "DTLSSocket/tinydtls/session.c",
+                     "DTLSSocket/tinydtls/aes/rijndael.c",
+                     "DTLSSocket/tinydtls/sha2/sha2.c"
+                     ],
+                    include_dirs=['DTLSSocket/tinydtls'],
+                    define_macros=[('DTLSv12', '1'),
+                                   ('WITH_SHA256', '1'),
+                                   ('DTLS_CHECK_CONTENTTYPE', '1'),
+                                   ('_GNU_SOURCE', '1')],
+                    undef_macros = [ "NDEBUG" ],
+                    )])
+
+ext_modules = get_ext_modules()
 
 setup(
     name="DTLSSocket",
@@ -47,7 +67,6 @@ setup(
     url         = "https://git.fslab.de/jkonra2m/tinydtls-cython",
     py_modules  = [ "DTLSSocket.DTLSSocket"],
     cmdclass    = {"build_ext": prepare_tinydtls},
-    ext_modules = cy_build,
-    setup_requires = [ "Cython==0.27.2" ],
-    install_requires = [ "Cython==0.27.2" ],
+    ext_modules = ext_modules,
+    setup_requires = [ CYTHON_VERSION ],
     )