chore(deps): update dependency google-cloud-spanner to v3.7.0 (#473)

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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) | `==3.6.0` -> `==3.7.0` | [![age](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.7.0/age-slim)](https://docs.renovatebot.com/merge-confidence/) | [![adoption](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.7.0/adoption-slim)](https://docs.renovatebot.com/merge-confidence/) | [![passing](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.7.0/compatibility-slim/3.6.0)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://badges.renovateapi.com/packages/pypi/google-cloud-spanner/3.7.0/confidence-slim/3.6.0)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

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

### [`v3.7.0`](https://togithub.com/googleapis/python-spanner/blob/master/CHANGELOG.md#&#8203;370-httpswwwgithubcomgoogleapispython-spannercomparev360v370-2021-07-29)

[Compare Source](https://togithub.com/googleapis/python-spanner/compare/v3.6.0...v3.7.0)

##### Features

-   add always_use_jwt_access ([#&#8203;381](https://www.github.com/googleapis/python-spanner/issues/381)) ([0f1a5de](https://www.github.com/googleapis/python-spanner/commit/0f1a5ded572685a96d29a60c959cb00a48f7a87f))
-   add configurable leader placement support ([#&#8203;399](https://www.github.com/googleapis/python-spanner/issues/399)) ([7f1b120](https://www.github.com/googleapis/python-spanner/commit/7f1b1209e62062014545cf959d41f04184552eec))
-   add sample for low cost instances ([#&#8203;392](https://www.github.com/googleapis/python-spanner/issues/392)) ([3f4f93f](https://www.github.com/googleapis/python-spanner/commit/3f4f93f75f5585a82047bf8d83a24622ad776ecb))

##### Bug Fixes

-   avoid bad version of `opentelemetry-instrumentation` ([#&#8203;429](https://www.github.com/googleapis/python-spanner/issues/429)) ([1620c12](https://www.github.com/googleapis/python-spanner/commit/1620c12a56e0d007cf010690bab303db06d0c914))
-   **deps:** pin 'google-{api,cloud}-core' to allow 2.x versions ([#&#8203;415](https://www.github.com/googleapis/python-spanner/issues/415)) ([b0455d0](https://www.github.com/googleapis/python-spanner/commit/b0455d0ab657cd053a7527e99bdbfadc4de23b30))
-   disable always_use_jwt_access ([c37bf21](https://www.github.com/googleapis/python-spanner/commit/c37bf21afdf417757eff67fe8500aa65f49fd5ad))
-   disable always_use_jwt_access ([#&#8203;395](https://www.github.com/googleapis/python-spanner/issues/395)) ([c37bf21](https://www.github.com/googleapis/python-spanner/commit/c37bf21afdf417757eff67fe8500aa65f49fd5ad))
-   enable self signed jwt for grpc ([#&#8203;427](https://www.github.com/googleapis/python-spanner/issues/427)) ([2487800](https://www.github.com/googleapis/python-spanner/commit/2487800e31842a44dcc37937c325e130c8c926b0))
-   support merging for NUMERIC values ([#&#8203;434](https://www.github.com/googleapis/python-spanner/issues/434)) ([06b4215](https://www.github.com/googleapis/python-spanner/commit/06b4215f76ae806eba1d0d07115c8c90b8c7482d)), closes [#&#8203;433](https://www.github.com/googleapis/python-spanner/issues/433)

##### Documentation

-   fix docstring for session.py ([#&#8203;387](https://www.github.com/googleapis/python-spanner/issues/387)) ([3132587](https://www.github.com/googleapis/python-spanner/commit/3132587453f7bd0be72ebc393626b5c8b1bab982))

</details>

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samples/samples/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==3.6.0
+google-cloud-spanner==3.7.0
 futures==3.3.0; python_version < "3"