chore(deps): update dependency pandas-gbq to v0.15.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
pandas-gbq	==0.14.1 -> ==0.15.0

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Release Notes

pydata/pandas-gbq

v0.15.0

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Features

• Load DataFrame with to_gbq to a table in a project different from
  the API client project. Specify the target table ID as
  project.dataset.table to use this feature. (#​321, #​347)
• Allow billing project to be separate from destination table project
  in to_gbq. (#​321)

Bug fixes

• Avoid 403 error from to_gbq when table has policyTags. (#​354)
• Avoid client.dataset deprecation warnings. (#​312)

Dependencies

• Drop support for Python 3.5 and 3.6. (#​337)
• Drop support for google-cloud-bigquery==2.4.* due to query
  hanging bug. (#​343)

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