avro

AVRO-1751: Add python3 compatibility (#979)

* AVRO-1751: add python3 compatibility

Keeping in line with the same logic present in the avro java library
that handles type promotions, this aims to be a faithful adaptation of
that code in python.

Most tests were also ported, with the exception of the ones dealing with
enum default, since the python api does not support that yet

See https://github.com/apache/avro/blob/master/lang/java/avro/src/main/java/org/apache/avro/SchemaCompatibility.java

* typing: fix mypy issues

Merge is now a function instead of a SchemaCompatibilityResult method to
circumvent the fact that a method cannot receive type hints of it's own
type without __future__ imports until 3.7 an without until 3.10

More at https://www.python.org/dev/peps/pep-0563/

* style: make compat code python3.5 compatible

- remove f-string
- remove class member annotations

Co-authored-by: Gabriel Tincu <gabi@aiven.io>