Switched back to the Zenodo badge image

There was a lack of communication from the Zenodo team, but [this issue](zenodo/zenodo#2165) was closed and the problem seems to be fixed.

README.md

@@ -6,7 +6,7 @@
 [![Build Status](https://dev.azure.com/cadquery/conda-packages/_apis/build/status/CadQuery.cadquery?branchName=master)](https://dev.azure.com/cadquery/conda-packages/_build/latest?definitionId=2&branchName=master)
 [![codecov](https://codecov.io/gh/CadQuery/cadquery/branch/master/graph/badge.svg)](https://codecov.io/gh/CadQuery/cadquery)
 [![Documentation Status](https://readthedocs.org/projects/cadquery/badge/?version=latest)](https://cadquery.readthedocs.io/en/latest/?badge=latest)
-[![DOI](https://7bindustries.com/static/images/zenodo.4498634.svg)](https://doi.org/10.5281/zenodo.4498634)
+[![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.4498634.svg)](https://doi.org/10.5281/zenodo.4498634)
 
 
 ## What is CadQuery