chore(deps): update dependency matplotlib to v3.3.4 (googleapis#495)

Author: renovate-bot

samples/snippets/requirements.txt

@@ -4,7 +4,7 @@ google-auth-oauthlib==0.4.2
 grpcio==1.35.0
 ipython==7.16.1; python_version < '3.7'
 ipython==7.17.0; python_version >= '3.7'
-matplotlib==3.3.3
+matplotlib==3.3.4
 pandas==1.1.5; python_version < '3.7'
 pandas==1.2.0; python_version >= '3.7'
 pyarrow==3.0.0