Update dependency google-cloud-pubsub to v1.6.0 [(#4039)](GoogleCloudPlatform/python-docs-samples#4039)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-pubsub](https://togithub.com/googleapis/python-pubsub) | minor | `==1.5.0` -> `==1.6.0` |

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### Release Notes

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

### [`v1.6.0`](https://togithub.com/googleapis/python-pubsub/blob/master/CHANGELOG.md#&#8203;160-httpswwwgithubcomgoogleapispython-pubsubcomparev150v160-2020-06-09)

[Compare Source](https://togithub.com/googleapis/python-pubsub/compare/v1.5.0...v1.6.0)

##### Features

-   Add flow control for message publishing ([#&#8203;96](https://www.github.com/googleapis/python-pubsub/issues/96)) ([06085c4](https://www.github.com/googleapis/python-pubsub/commit/06085c4083b9dccdd50383257799904510bbf3a0))

##### Bug Fixes

-   Fix PubSub incompatibility with api-core 1.17.0+ ([#&#8203;103](https://www.github.com/googleapis/python-pubsub/issues/103)) ([c02060f](https://www.github.com/googleapis/python-pubsub/commit/c02060fbbe6e2ca4664bee08d2de10665d41dc0b))

##### Documentation

-   Clarify that Schedulers shouldn't be used with multiple SubscriberClients ([#&#8203;100](https://togithub.com/googleapis/python-pubsub/pull/100)) ([cf9e87c](https://togithub.com/googleapis/python-pubsub/commit/cf9e87c80c0771f3fa6ef784a8d76cb760ad37ef))
-   Fix update subscription/snapshot/topic samples ([#&#8203;113](https://togithub.com/googleapis/python-pubsub/pull/113)) ([e62c38b](https://togithub.com/googleapis/python-pubsub/commit/e62c38bb33de2434e32f866979de769382dea34a))

##### Internal / Testing Changes

-   Re-generated service implementaton using synth: removed experimental notes from the RetryPolicy and filtering features in anticipation of GA, added DetachSubscription (experimental) ([#&#8203;114](https://togithub.com/googleapis/python-pubsub/pull/114)) ([0132a46](https://togithub.com/googleapis/python-pubsub/commit/0132a4680e0727ce45d5e27d98ffc9f3541a0962))
-   Incorporate will_accept() checks into publish() ([#&#8203;108](https://togithub.com/googleapis/python-pubsub/pull/108)) ([6c7677e](https://togithub.com/googleapis/python-pubsub/commit/6c7677ecb259672bbb9b6f7646919e602c698570))

</details>

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samples/api-client/manager/requirements.txt

@@ -4,6 +4,6 @@ google-api-python-client==1.9.3
 google-auth-httplib2==0.0.3
 google-auth==1.18.0
 google-cloud-iot==1.0.0
-google-cloud-pubsub==1.5.0
+google-cloud-pubsub==1.6.0
 paho-mqtt==1.5.0
 pyjwt==1.7.1