Remove avro-python3 dependency from Beam

Beam has been using fastavro as default dependency for Avro IO on Python3 and no issues have been reported. Fastavro has more frequent release cycle and is faster.

Let's remove the dependency on avro-python3 to avoid branching in our codebase so that we don't replicate it in other parts of Beam, which was recently happening on:

https://github.com/apache/beam/pull/15185#pullrequestreview-729352200
https://github.com/apache/beam/pull/15185#pullrequestreview-731157837