Ineffective security measure in copy_file.py

This line of code is intended to purge the secret from memory:

https://github.com/aws-samples/aws-secrets-manager-ssh-key-rotation/blob/163b90fd217a7ce9285d700bca8afe87a0f181b3/scripts/copy_file.py#L65

Unfortunately, it does not: python strings are immutable, and reassignment of a string variable in python allocates a new buffer, leaving the old buffer in memory as garbage. Thus the best practices being espoused here are not actually being adhered to.

I verified this using a similar approach to [this nice blog post](https://www.sjoerdlangkemper.nl/2016/06/09/clearing-memory-in-python/):

```
$ echo 'import os; x="verysecret"; x="###############"; del x; os.abort()' > test.py
$ ulimit -c unlimited
$ python3 test.py
Aborted (core dumped)
$ grep verysecret core
Binary file core matches
```

Thus this string would remain in RAM until the same address happens to be used again later.

The [AWS blog post](https://aws.amazon.com/blogs/security/how-to-use-aws-secrets-manager-securely-store-rotate-ssh-key-pairs/) accompanying this repository wrote:

> details about keeping secret data in volatile memory will follow later in this post.

...but no such details seemed to be forthcoming.

Start with scripts/copy_file.py at the linked line 65 and reproduce the memory-retention behavior using the issue's Python example. Review the accompanying AWS blog post and determine a supported way to handle the secret without relying on immutable-string reassignment; done means the ineffective security measure is removed or replaced and the behavior is verified.

Written by the indexing model from the issue text.