avro

AVRO-3097: Run Schema Compatibility checks as part of Unittest (#1166)

* AVRO-3097 Run Schema Compatibility checks as part of Unittest

Test methods in `test_compatibility.py` were being ignored in
unittest runs because they were not connected to unittest.TestCase

This commit encloses the test cases within a class deriving from
`unittest.TestCase` and also changes assert statements to be
in compliance with unittest.

* AVRO-3097: Expose `compatibility` module as part of avro package

`compatibility` package was not accessible through the avro package.
This PR exposes it and allows `import avro.compatibility`.