[setup.py] get_version: replace `importlib` with raw text scanning

[zuoxingdong]

e.g.

# only read through the raw file without importing any modules
def _get_version(dirpath):
    var_name = '__version__'
    file_path = Path(dirpath)/'__init__.py'
    with open(file_path, 'r') as fp:
        for line in fp:
            if line.startswith(var_name):
                g = {}
                exec(line, g)
                return g[var_name]
        raise ValueError(f'`{var_name}` must be defined within `{file_path.as_posix()}`')