The RDKit is an open-source cheminformatics toolkit that has had Python 3 support for several years and has recently committed to move over completely to Python 3 by 2020:
http://www.mail-archive.com/rdkit-discuss@lists.sourceforge.net/msg06935.html

Here's the text:
- [![](assets/rdkit.png)RDKit](https://github.com/rdkit/rdkit)

and the logo:

And the timeline (hopefully I got this right):

'RDKit':[
	    {content: '2014.9.1 - 2019.03.1', start: '2014-09-1', end: '2019-09-1', py2:true},
	    {content: '2019.9.1 -' , start: '2019-09-01', end: '2021-03-01'}
	    ]