chore(deps): update dependency google-cloud-spanner to v3.8.0 (#522)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-spanner](https://togithub.com/googleapis/python-spanner) | `==3.7.0` -> `==3.8.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.8.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.8.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.8.0/compatibility-slim/3.7.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.8.0/confidence-slim/3.7.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>googleapis/python-spanner</summary>

### [`v3.8.0`](https://togithub.com/googleapis/python-spanner/blob/master/CHANGELOG.md#&#8203;380-httpswwwgithubcomgoogleapispython-spannercomparev370v380-2021-08-15)

[Compare Source](https://togithub.com/googleapis/python-spanner/compare/v3.7.0...v3.8.0)

##### Features

-   use DML batches in `executemany()` method ([#&#8203;412](https://www.togithub.com/googleapis/python-spanner/issues/412)) ([cbb4ee3](https://www.github.com/googleapis/python-spanner/commit/cbb4ee3eca9ac878b4f3cd78cfcfe8fc1acb86f9))

##### Bug Fixes

-   **samples:** batch_update() results processing error ([#&#8203;484](https://www.togithub.com/googleapis/python-spanner/issues/484)) ([bdd5f8b](https://www.github.com/googleapis/python-spanner/commit/bdd5f8b201d1b442837d4fca1d631fe171e276b9))

</details>

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samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.7.0
+google-cloud-spanner==3.8.0
 futures==3.3.0; python_version < "3"