Timestamp Returned In Scientific Notation By API Crashes Field Cast

[pkenjora]

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Summary of the issue

Context
Write a timestamp field to the BigQuery API that was written using UnixTime stamp from python.
Then try to load it as an object.

Expected Behavior:
Timestamp comes back without errors.

Actual Behavior:
Getting a TypeError when int tries to parse "1.67899e8".

API client name and version

google-cloud-bigquery

Reproduction steps: code

See: google-cloud-bigquery/google/cloud/bigquery/_helpers.py#L264

Line 264 is: return _datetime_from_microseconds(int(value))

The int cast causes the error when the API returns '1.345345e8', which it does.

Reproduction steps: supporting files

Write a timestamp to bigqurry using time.time()... its accepted.
Load the value back.... causes error.

Reproduction steps: actual results

Reproduction steps: expected results

### OS & version + platform

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### Python environment

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### Python dependencies

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### Additional context

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