chore(deps): update dependency sendgrid to v6.4.1 (#4157)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [sendgrid](https://togithub.com/sendgrid/sendgrid-python) | minor | `==6.3.1` -> `==6.4.1` |

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### Release Notes

<details>
<summary>sendgrid/sendgrid-python</summary>

### [`v6.4.1`](https://togithub.com/sendgrid/sendgrid-python/blob/master/CHANGELOG.md#&#8203;2020-06-25-Version-641)

[Compare Source](https://togithub.com/sendgrid/sendgrid-python/compare/6.4.0...6.4.1)

**Library - Fix**

-   [PR #&#8203;914](https://togithub.com/sendgrid/sendgrid-python/pull/914): add dependency to install requires. Thanks to [@&#8203;eshanholtz](https://togithub.com/eshanholtz)!

### [`v6.4.0`](https://togithub.com/sendgrid/sendgrid-python/blob/master/CHANGELOG.md#&#8203;2020-06-24-Version-640)

[Compare Source](https://togithub.com/sendgrid/sendgrid-python/compare/6.3.1...6.4.0)

**Library - Docs**

-   [PR #&#8203;912](https://togithub.com/sendgrid/sendgrid-python/pull/912): added docstrings to Stats classes. Thanks to [@&#8203;DougCal](https://togithub.com/DougCal)!

**Library - Feature**

-   [PR #&#8203;908](https://togithub.com/sendgrid/sendgrid-python/pull/908): add support for dynamic template data to Email class. Thanks to [@&#8203;childish-sambino](https://togithub.com/childish-sambino)!
-   [PR #&#8203;901](https://togithub.com/sendgrid/sendgrid-python/pull/901): verify signature from event webhook. Thanks to [@&#8203;eshanholtz](https://togithub.com/eshanholtz)!

**Library - Fix**

-   [PR #&#8203;904](https://togithub.com/sendgrid/sendgrid-python/pull/904): revert "feat: Add equality to Email". Thanks to [@&#8203;childish-sambino](https://togithub.com/childish-sambino)!

</details>

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appengine/flexible/sendgrid/requirements.txt

@@ -1,3 +1,3 @@
 Flask==1.1.2
-sendgrid==6.3.1
+sendgrid==6.4.1
 gunicorn==20.0.4

appengine/standard/sendgrid/requirements.txt

@@ -1 +1 @@
-sendgrid==6.3.1
+sendgrid==6.4.1