chore(deps): update dependency matplotlib to v3.2.1 [(#3180)](GoogleCloudPlatform/python-docs-samples#3180)

Co-authored-by: Leah E. Cole <[email protected]>

Author: 2 people

samples/snippets/requirements.txt

@@ -1,5 +1,6 @@
 google-cloud-bigquery[pandas]==1.20.0
 google-auth-oauthlib==0.4.0
 ipython==7.8.0
-matplotlib==3.1.1
+matplotlib==3.2.1
 pytz==2019.3
+