gh-151039: Fix a crash when datetime.timedelta outlives _datetime module

[sobolevn]

Several changes:

1. Always check that GET_CURRENT_STATE can return NULL
2. Unwrap (void)PyWeakref_GetRef call into a full check for safety, there are multiple things that can go wrong when getting a weakref
3. Remove assert(!PyErr_Occurred()); followed by if (PyErr_Occurred()) {

• Issue: Crash: NULL deref in _datetime when a static type outlives its module #151039

[sobolevn]

Ok, I just discovered #132413 which is related.