Bumped boto3 version uppper range for dbt-redshift

[tyang209]

resolves #2931

Description

The new Pip dependency resolver as of version 20.3 causes mismatched dependencies to raise an error when installing requirements. This should alleviate some issues by raising the upper range of what is allowed for boto3 to 1.16 which matches what python-snowflake-connector 2.3.6.

Checklist

• I have signed the CLA
• 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
• I have updated the CHANGELOG.md and added information about my change to the "dbt next" section.

Additional notes.

It looks like the snowflake-connector also removed botocore dependency as of 2.2.6 Link

I thought about removing it from dbt-redshift also but figured I should ask.

I am not 100% sure about testing requirements. I'm just dipping my toe for the first time in doing OSS contributions! I did run make test-quick locally and it did pass all integration tests but failed RPC tests.

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[jtcohen6]

Looks great, @tyang209! Could you add yourself as a contributor? :)

[tyang209]

Looks great, @tyang209! Could you add yourself as a contributor? :)

Done!