tasks(js_modules, sass): remove support for `npm`: `npm` cause trouble during installation, and it will be resource cousuming to find the source and maintain it in addition to `yarn`, thus `yarn` is for now kept and the only supported NodeJS package manager.

import os.path

version_file = os.path.join(os.path.dirname(__file__), "VERSION")
try:
    with open(version_file) as f:
        __version__ = f.read().strip()
except NotImplementedError:
    # pyjamas workaround
    __version__ = "0.7.0D"