python string conversion

- use _PyUnicode_AsStringAndSize where possible
- use %R for PyErr_Format(...) rather then running repr on the object explicitly 
- use const char

source/blender/imbuf/intern/moviecache.c

@@ -230,7 +230,7 @@ void IMB_moviecache_put(MovieCache *cache, void *userkey, ImBuf *ibuf)
 
 	key= BLI_mempool_alloc(cache->keys_pool);
 	key->cache_owner= cache;
-	key->userkey= BLI_mempool_alloc(cache->userkeys_pool);;
+	key->userkey= BLI_mempool_alloc(cache->userkeys_pool);
 	memcpy(key->userkey, userkey, cache->keysize);
 
 	item= BLI_mempool_alloc(cache->items_pool);