Unfortunately it looks like there is no progress on converting to python3. 
I found this tool which maybe helps:
https://docs.python.org/3/library/2to3.html
And there is also an “online” converter:
https://python2to3.com/
Maybe we can do something with this converter?
Hello,
this tool only does the trivial part of the port to python3.
You should better start either from :
https://github.com/ccrisan/motioneye/pull/1805
This pull request is much more advanced than the python3 branch of motioneye.
or from :
https://github.com/jmichault/motioneye.eo
which is my own fork and work correctly on my RasPi3 for my network cameras.
I have also made recently some work on USB V4L cameras.
Sounds good! If i can help you let me know (if i can, i have not really coding skills). I have MMAL, USB and network cameras.