chore(deps): update dependency matplotlib to v3.3.1 (#224)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [matplotlib](https://matplotlib.org) ([source](https://togithub.com/matplotlib/matplotlib)) | patch | `==3.3.0` -> `==3.3.1` |

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### Release Notes

<details>
<summary>matplotlib/matplotlib</summary>

### [`v3.3.1`](https://togithub.com/matplotlib/matplotlib/releases/v3.3.1)

[Compare Source](https://togithub.com/matplotlib/matplotlib/compare/v3.3.0...v3.3.1)

This is the first bugfix release of the 3.3.x series.

This release contains several critical bug-fixes:

-   fix docstring import issues when running Python with optimization
-   fix `hist` with categorical data, such as with Pandas
-   fix install on BSD systems
-   fix nbagg compatibility with Chrome 84+
-   fix ordering of scatter marker size in 3D plots
-   fix performance regression when plotting `Path`s
-   fix reading from URL in `imread`
-   fix several regressions with new date epoch handling
-   fix some bad constrained and tight layout interactions with colorbars
-   fix use of customized toolbars in TkAgg and WXAgg backends

</details>

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samples/snippets/requirements.txt

@@ -2,5 +2,5 @@ google-cloud-bigquery[pandas,bqstorage,pyarrow]==1.26.1
 google-auth-oauthlib==0.4.1
 ipython==7.16.1; python_version < '3.7'
 ipython==7.17.0; python_version >= '3.7'
-matplotlib==3.3.0
+matplotlib==3.3.1
 pytz==2020.1