avro

Modernize example python (#2935)

https://github.com/apache/avro/pull/2932 fixed one problem, but there were others:

1. The existence of a "WTF" field in the record that is not in the schema, causing the code to fail. It's not reflected in [the actual docs](https://avro.apache.org/docs/1.11.1/getting-started-python/#serializing-and-deserializing-without-code-generation).
2. The use of text mode for writing binary data. (This is also correct in the Getting Started docs.)
3. Using strings for paths instead of Path objects. This is a minor thing, but a tidy practice.
4. The possibility of a crash causing a resource leak because our example doesn't use context managers to close the file. (This is also _incorrect_ in the Getting Started docs.)