Python3 migration & Emscripten update

[jmarcosfer]

Two main changes to the build pipeline before other development tasks:

• update scripts to Python 3
• update to more recent version of Emscripten (1.39.19 --> 3.1.24)

Notes:

• Since Emscripten 2.0.3 the default output format is executable javascript (+ wasm) instead of compiling to object files (.bc), unless a -c, -r, or -shared flag is given. Therefore commands have changed slightly and we have two options: either (1) build to object files first, like we used to do, but individually for bindings_essentiajs.o and essentiajs.o because now it doesn't accept multiple files with -o flag; (2) use the new default way, which also means fewer commands, and go directly from C++ to JS+WASM.
• Not sure whether to leave the changes on "update included and excluded algo lists" and "add excluded tensor algos to script", perhaps those should go in on a separate development branch.

[jmarcosfer]

@albincorreya let me know what you think about the 2 notes in question, please

[jmarcosfer]

The new default compiling method looks clean. Is there any major performance difference between these methods?

I assume we will migrate the python scripts to python3 in another PR?

I don't see any noticeable performance difference between the two methods, so I'll stick to the newer default way, which also looks cleaner as you say :) The change to Python3 should be included in this PR, that was the initial goal of this PR, but then along the way I found I should update Emscripten too. I guess it's okay to include the two as updating the build pipeline.