chore(deps): update dependency google-cloud-spanner to v3.4.0 (#330)

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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) | `==3.3.0` -> `==3.4.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.4.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.4.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.4.0/compatibility-slim/3.3.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.4.0/confidence-slim/3.3.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v3.4.0`](https://togithub.com/googleapis/python-spanner/blob/master/CHANGELOG.md#&#8203;340-httpswwwgithubcomgoogleapispython-spannercomparev330v340-2021-04-29)

[Compare Source](https://togithub.com/googleapis/python-spanner/compare/v3.3.0...v3.4.0)

##### Features

-   add samples for CMEK support ([#&#8203;275](https://www.github.com/googleapis/python-spanner/issues/275)) ([f8d9bd3](https://www.github.com/googleapis/python-spanner/commit/f8d9bd33e04675a8dca148c2fae4a9133beebbca))
-   added support for numeric field for python decimal value ([#&#8203;316](https://www.github.com/googleapis/python-spanner/issues/316)) ([070a171](https://www.github.com/googleapis/python-spanner/commit/070a1712dc34afb68105194060bb2fe6177fbac5))
-   **dbapi:** remove string conversion for numeric fields ([#&#8203;317](https://www.github.com/googleapis/python-spanner/issues/317)) ([772aa3c](https://www.github.com/googleapis/python-spanner/commit/772aa3c2ffbdf3f863c09db176697b3ad70adbcf))

##### Bug Fixes

-   correctly set resume token when restarting streams ([#&#8203;314](https://www.github.com/googleapis/python-spanner/issues/314)) ([0fcfc23](https://www.github.com/googleapis/python-spanner/commit/0fcfc2301246d3f20b6fbffc1deae06f16721ec7))
-   support INSERT from SELECT clause with args ([#&#8203;306](https://www.github.com/googleapis/python-spanner/issues/306)) ([0dcda5e](https://www.github.com/googleapis/python-spanner/commit/0dcda5e21f8fb30ee611fddf0829684d86ced0ef))

##### Performance Improvements

-   use protobuf for metadata to reduce type conversions ([#&#8203;325](https://www.github.com/googleapis/python-spanner/issues/325)) ([5110b9b](https://www.github.com/googleapis/python-spanner/commit/5110b9bc31804db9777a23fca60360119840640c))

</details>

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samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.3.0
+google-cloud-spanner==3.4.0
 futures==3.3.0; python_version < "3"