rust-cpython: mark PySharedState as Sync so &'PySharedState can be Send...
rust-cpython: mark PySharedState as Sync so &'PySharedState can be Send
The goal is to store &'static PySharedState in $leaked struct, which allows
us to move the $leaked struct out of the macro. Currently, it depends on
inner.$data_member(py), which can't be generalized.
PySharedState is Sync because any mutation or read operation is synchronized
by the Python GIL, py: Python<'a>, which should guarantee that &'PySharedState
can be sent to another thread.
