We keep the #[link] attributes in #[cfg_attr(windows)] so that we don't require a nightly Rust build on non-Windows platforms.
This can be simplified once RFC 1717 is available in a stable rust version.
This commit also increases the minimum Rust version to 1.13.
Since the `Python` token no longer is a part of `PyObject`,
lots of methods now require the token as additional argument.
This [breaking-change] breaks everything!
Nikolay Kim