chore(deps): update dependency google-cloud-spanner to v3.20.0 (#809)

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This PR contains the following updates:

| Package | Change | Age | Adoption | Passing | Confidence |
|---|---|---|---|---|---|
| [google-cloud-spanner](https://github.com/googleapis/python-spanner) | `==3.19.0` -> `==3.20.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.20.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.20.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.20.0/compatibility-slim/3.19.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.20.0/confidence-slim/3.19.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v3.20.0`](https://github.com/googleapis/python-spanner/blob/HEAD/CHANGELOG.md#&#8203;3200-httpsgithubcomgoogleapispython-spannercomparev3190v3200-2022-08-30)

[Compare Source](https://github.com/googleapis/python-spanner/compare/v3.19.0...v3.20.0)

##### Features

-   Adds TypeAnnotationCode PG_JSONB ([#&#8203;792](https://github.com/googleapis/python-spanner/issues/792)) ([6a661d4](https://github.com/googleapis/python-spanner/commit/6a661d4492bcb77abee60095ffc2cfdc06b48124))

##### Bug Fixes

-   if JsonObject serialized to None then return `null_value` instead of `string_value` ([#&#8203;771](https://github.com/googleapis/python-spanner/issues/771)) ([82170b5](https://github.com/googleapis/python-spanner/commit/82170b521f0da1ba5aaf064ba9ee50c74fe21a86))

</details>

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samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.19.0
+google-cloud-spanner==3.20.0
 futures==3.3.0; python_version < "3"