minor bug fixes for python3 + .c++ file support that is not destructive

capnp/templates/setup.py.tmpl

@@ -1,6 +1,7 @@
 #!/usr/bin/env python
 from distutils.core import setup
 from Cython.Build import cythonize
+from shutil import copyfile
 import os
 import re
 
@@ -10,7 +11,11 @@ files = [{% for f in code.requestedFiles %}"{{f.filename}}", {% endfor %}]
 for f in files:
   cpp_file = f + '.cpp'
   cplus_file = f + '.c++'
-  cpp_mod = os.path.getmtime(cpp_file)
+  cpp_mod = 0
+  try:
+    cpp_mod = os.path.getmtime(cpp_file)
+  except:
+    pass
   cplus_mod = 0
   try:
     cplus_mod = os.path.getmtime(cplus_file)
@@ -19,7 +24,7 @@ for f in files:
   if not os.path.exists(cpp_file) or cpp_mod < cplus_mod:
     if not os.path.exists(cplus_file):
       raise RuntimeError("You need to run `capnp compile -oc++` in addition to `-ocython` first.")
-    os.rename(cplus_file, cpp_file)
+    copyfile(cplus_file, cpp_file)
 
     with open(f + '.h', "r") as file:
         lines = file.readlines()