Prep bigquery-0.25.0 release.

[tseaver]

Requires merge of #3526 to bump core to 0.25.0.

Draft release notes:

google-cloud-bigquery-0.25.0

• Update google-cloud-core dependency to ~= 0.25.
• Fix display of _EnumProperty ValueError messages. (PR BigQuery _EnumProperty ValueError messages are not displayed properly #3520)
• Make QueryResponse.fetch_data return an iterator. (PR Make 'QueryResponse.fetch_data' return an iterator. #3484, issue BigQuery: fetch_data() should return a core iterator #2840)
• Add read-only Query.num_dml_affected_rows property. (PR Add 'Query.num_dml_affected_rows' property. #3460, issue BigQuery: expose numDmlAffectedRows in query results #2920)
• Marshal row data correctly in Table.insert_data() (PR Marshal row data correctly in 'Table.insert_data()' #3426, issue 'Table.insert_data' does not support Datetime values #2957)
• Fix support for loading AVRO files from the local filesystem (PR Loading AVRO files from local filesystem. #3427, issue Schema needed when uploading AVRO file from local filesystem, but not from GCS. #3416)
• Fix support for creating partitioned tables. (PR Fix new partitioned table #3381)
• Update BigQuery Query parameter reference to better indicate that timeout_ms sets timeout on the request, not on the query. (PR Update BQ Query parameter reference #3320, issue timeout_ms docs are misleading #3317)

---

Not included in notes:

• Re-enable pylint in info-only mode for all packages (Re-enable pylint in info-only mode for all packages #3519)
• Reload BigQuery table in system test before fetching data. (Reloading BigQuery table in system test before fetching data. #3468)
• Vision semi-GAPIC (Vision semi-GAPIC #3373)
• Use binary streams for uploads in BigQuery system tests. (PR Using binary streams for uploads in BigQuery system tests. #3374, issue BigQuery system test broken by google-resumable-media #3371)
• Adding check that all setup.py README's are valid RST. (Adding check that **all** setup.py README's are valid RST. #3318)
• Fix repeated spelling error (Fix repeated spelling error #3260)

[dhermes]

LGTM