Bump plotly from 1.9.10 to 4.7.1

Bumps [plotly](https://github.com/plotly/plotly.py) from 1.9.10 to 4.7.1.
- [Release notes](https://github.com/plotly/plotly.py/releases)
- [Changelog](https://github.com/plotly/plotly.py/blob/master/CHANGELOG.md)
- [Commits](https://github.com/plotly/plotly.py/commits/v4.7.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements/base.txt

@@ -21,7 +21,7 @@ gocardless_pro==1.1.0
 jsonfield==1.0.3
 markdown==2.6.2
 Pillow==4.3.0
-plotly==1.9.10
+plotly==4.7.1
 psycopg2-binary==2.7.4
 
 sorl-thumbnail==12.6.3