Add dbt-core install back to dev-requirements.txt

[colin-rogers-dbt]

• add dbt-core back to dev-requirements.txt

resolves #1080
docs dbt-labs/docs.getdbt.com/#

Problem

Solution

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

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the dbt-bigquery contributing guide.

[mikealfare]

@colin-rogers-dbt The failed tests are due to validating against the adapter version instead of the dbt-core version. Here's the fix in dbt-adapters, though it would need to be released for this CI to pick it up, unless we point this CI to GitHub.

[mikealfare]

I think we also want the dev version of dbt-adapters here too, right?