chore(deps): update dependency dialogflow to v0.8.0 [(#3147)](GoogleCloudPlatform/python-docs-samples#3147)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [dialogflow](https://togithub.com/dialogflow/dialogflow-python-client-v2) | minor | `==0.7.2` -> `==0.8.0` |

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### Release Notes

<details>
<summary>dialogflow/dialogflow-python-client-v2</summary>

### [`v0.8.0`](https://togithub.com/dialogflow/dialogflow-python-client-v2/blob/master/CHANGELOG.md#&#8203;080httpswwwgithubcomgoogleapisdialogflow-python-client-v2comparev072v080-2020-03-26)

[Compare Source](https://togithub.com/dialogflow/dialogflow-python-client-v2/compare/v0.7.2...v0.8.0)

##### Features

-   **dialogflow:** add `MediaContent`,  `BrowseCarouselCard`, `ColumnProperties`in v2; add `SpeechContext`, `SpeechWordInfo`in v2; add `enable_word_info`, `speech_contexts`,`model` to InputAudioConfig in v2; add `subtitles` to `Intent.Message.ListSelect` in `v2beta1`; add `language_code` to `ListKnowledgeBase` in v2beta1; add `webhook_headers` to `QueryParameters` in v2beta1 ([#&#8203;175](https://www.github.com/googleapis/dialogflow-python-client-v2/issues/175)) ([713846b](https://www.github.com/googleapis/dialogflow-python-client-v2/commit/713846b7ed99eaf78cdf383aa9d39b43731b9a0d))
-   add validation support to v2; add `output_audio_config_mask` to `detect_intent` method in v2beta1 and v2; add sub agent to v2beta1 (via synth) ([#&#8203;179](https://www.github.com/googleapis/dialogflow-python-client-v2/issues/179)) ([5a6f18e](https://www.github.com/googleapis/dialogflow-python-client-v2/commit/5a6f18e362b5dd87affbe75e0d0bfc0c21ab87a9))

##### [0.7.2](https://www.github.com/googleapis/dialogflow-python-client-v2/compare/v0.7.1...v0.7.2) (2019-10-18)

##### Bug Fixes

-   define version once in setup.py ([#&#8203;158](https://www.github.com/googleapis/dialogflow-python-client-v2/issues/158)) ([bf42fc4](https://www.github.com/googleapis/dialogflow-python-client-v2/commit/bf42fc45607b97bc040cbdacbde8ff5a4b6ad29b))

##### [0.7.1](https://www.github.com/googleapis/dialogflow-python-client-v2/compare/v0.7.0...v0.7.1) (2019-10-17)

##### Bug Fixes

-   pin google-api-core>=1.14.0, update format ([#&#8203;156](https://www.github.com/googleapis/dialogflow-python-client-v2/issues/156)) ([69951d0](https://www.github.com/googleapis/dialogflow-python-client-v2/commit/69951d013b7c99966848565e30d5ab1bad7229fb))

</details>

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samples/snippets/requirements.txt

@@ -1 +1 @@
-dialogflow==0.7.2
+dialogflow==0.8.0