Fix FFI cache race condition

The race condition exists in the following circumstance:

1. PyVEX process 1 parses ffi and opens a new cache file but has not
   written to it
2. PyVEX process 2 goes to parse ffi, sees the existing (empty) cache
   file, opens it, then fails when it attempts to read an empty file

This commit makes cache writing atomic, preventing the race condition.

pyvex/native.py

@@ -46,8 +46,11 @@ def _parse_ffi_str():
             "_declarations": ffi._parser._declarations,
             "_int_constants": ffi._parser._int_constants,
         }
-        with open(cache_location, "wb") as f:
+        # atomically write cache
+        temp_file = tempfile.NamedTemporaryFile(delete=False)
+        with open(temp_file.name, "wb") as f:
             f.write(pickle.dumps(cache))
+        os.replace(temp_file.name, cache_location)
 
 
 def _find_c_lib():