use custom JSON type for database for more generic support

[pmeier]

Offline report by @nenb. sqlalchemy.types.JSON supports

• PostgreSQL - see sqlalchemy.dialects.postgresql.JSON and sqlalchemy.dialects.postgresql.JSONB for backend-specific notes
• MySQL - see sqlalchemy.dialects.mysql.JSON for backend-specific notes
• SQLite as of version 3.9 - see sqlalchemy.dialects.sqlite.JSON for backend-specific notes
• Microsoft SQL Server 2016 and later - see sqlalchemy.dialects.mssql.JSON for backend-specific notes

Thus, using Ragna with a DB like snowflake is impossible.

This PR implements a custom Json type that augments types.String that does nothing but serializing incoming values and deserializing them again on the way out.

This breaks BC so we shouldn't include this in any potential bug fix release until 0.3.

[pmeier]

Lint errors are unrelated. See #390.