Replace build_requires with setup_requires keyword (GH-238)

setuptools doesn't have a `build_requires` keyword. By using the
`setup_requires` keyword, `pip install --use-pep517` will be able to
install Cython before building the package.

setup.py

@@ -491,7 +491,7 @@ def write_file(filename, content):
     ],
 
     packages=['lupa'],
-    build_requires=['Cython>=0.29.28'],
+    setup_requires=['Cython>=0.29.28'],
     ext_modules=ext_modules,
     libraries=ext_libraries,
     **extra_setup_args