Pipelinewise-target-snowflake

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*Changes*
- Update dependencies:
- snowflake-connector-python[pandas]
- boto3
- pytest
- python-dotenv

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*Changes*
- Revert use of `ujson`

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*Changes*
- Use `usjon` for JSON encoding/decoding

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*Fixes*
- Only drop pk constraint if table has one
- Don't raise `PrimaryKeyNotFoundException` when a record has a flasy pk value

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*Fixes*
- Respecting `flush_all_streams` when SCHEMA messages arrive.
- Improve logging for failed merge & copy queries.
- Drop NOT NULL constraint from primary key columns.
- Update PK constraints according to changes to SCHEMA's key properties.

*Changes*
- Dropping support for Python 3.6
- Adding support for Python 3.9
- Bump pytest to `7.1.1`
- Bump boto3 to `1.21`

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*Added*
- Support parallelism for table stages

*Fixes*
- Emit last encountered state message if there are no records.

*Changes*
- Migrate CI to github actions
- Bump dependencies