Suggest sorting of extension sources to ensure a reproducible build regardless of filesystem ordering.

re. https://github.com/pikepdf/pikepdf/pull/76

setup.py

@@ -21,7 +21,9 @@ class get_pybind_include(object):
 ext_modules = [
     Extension(
         'python_example',
-        ['src/main.cpp'],
+        # Sort input source files to ensure bit-for-bit reproducible builds
+        # (https://github.com/pybind/python_example/pull/53)
+        sorted(['src/main.cpp']),
         include_dirs=[
             # Path to pybind11 headers
             get_pybind_include(),