AttributeError: 'VarAccess' object has no attribute 'variable'

fix a bug where unpickling `annotated_vyper_module` would lead to
a crash:

```
AttributeError: 'VarAccess' object has no attribute 'variable'
```

this is a blocker for tooling, for instance, titanoboa
relies on pickle/unpickle to cache `CompilerData` objects:
https://github.com/vyperlang/titanoboa/blob/86df8936654db2068641/boa/util/disk_cache.py#L65-L66

the underlying issue is that `pickle.loads()` calls `obj.__hash__()`
for objects that are keys in a hashed data structure - namely, dicts,
sets and frozensets. this causes a crash when there is a cycle in
the object graph, because the object is not fully instantiated at the
time that `__hash__()` is called. this is a cpython issue, reported at
python/cpython#124937.

@serhiy-storchaka suggested the approach taken in this PR, which breaks
the loop before pickling:
https://github.com/python/cpython/issues/124937#issuecomment-2392227290

note that the implementation of `__reduce__()` in this PR is safe, since
there is no cycle in the hash function itself, since the recursion
breaks in `VarInfo.__hash__()`. in other words, there is no possibility
of `VarAccess.__hash__()` changing mid-way through reconstructing the
object.