Adjust __post_serialize__ signature

[gshank]

resolves #198

In support of serialization context, for dbt-core #10093.

Problem

The method signature of __post_serialize__ is changing to support serialization context.

Solution

We're adding the "context" keyword parameter to __post_serialize__.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX