Certificate path broken in Windows

[justinmichael-stripe]

This line assembles the path to the certificates bundle included with the library:

os.path.dirname(__file__), "data/ca-certificates.crt"

However, Windows uses backslashes (\) for path separators, not forward slashes (as hard-coded above) which results in this code triggering an invalid path error.

This line should be altered to provide an OS-agnostic path that works in all environments.