faster and cleaner rtd check for cython

setup.py

@@ -70,9 +70,9 @@ except ImportError:
     platform_freebsd_source = platform_freebsd_source.replace('.pyx', '.c')
     platform_darwin_source = platform_darwin_source.replace('.pyx', '.c')
     from distutils.command.build_ext import build_ext
-    if not all(os.path.exists(path) for path in [
+    if not on_rtd and not all(os.path.exists(path) for path in [
         compress_source, crypto_source, chunker_source, hashindex_source,
-        platform_linux_source, platform_freebsd_source]) and not on_rtd:
+        platform_linux_source, platform_freebsd_source]):
         raise ImportError('The GIT version of Borg needs Cython. Install Cython or use a released version.')