I’ve experimented with Transcrypt quite a bit. It is great for quick simple sketches, and remarkably fast / lightweight – so for writing JavaScript using Python syntax, it is excellent.
However, it isn’t for Python libraries. I quickly hit a wall when I failed trying to import non-core Python libraries (even relatively compact pure processing libraries such as pyparsing). So it is Python 3, but it really isn’t for Python integration any more than Python mode is – Python mode integrates with a subset of Python 2 libraries that Jython can crosscompile onto JVM, and Transcrypt integrates with what appears to be an even smaller subset of Python 3 libraries that it can crosscompile onto JavaScript.
For libraries, Pyodide looks very promising, as @villares mentioned – but it isn’t there yet, although there are early prototypes…