WriteToAvro fails if fastavro loads its python implementation of writer

It's possible for fastavro to fail to correctly load its cython implementation of the Writer class in which case it will silently fall back to a pure python implementation. If this happens there's no outward indication, but line 621 in io/avroio.py will fail because writer.fo is only present on the cython implementation.

To reproduce you can modify fastavro's write.py to only use its fallback:

#from . import _write

from . import _write_py as _write

And then run a workflow that sinks to WriteToAvro(use_fastavro=True).