Strip unwanted partition columns

[Jeremynadal33]

resolves #421

Description

Modify the function get_columns_in_relation so it does not return unwanted partitions related stuff like "Part 0", "Part 1", ... For more information on this behavior see 421

Checklist

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

FYI : There are currently a "fake" tests associated with this function. I can update it to add one but I need information on how to mock a BaseRelation object.

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[moomindani]

Thanks for your contribution.