do not support py3.8?

[zailaib]

Error detected while processing function virtualenv#activate: line 44: Traceback (most recent call last): File "", line 1, in File "/Users/xxx/.vim/plugged/vim-virtualenv/autoload/pyvenv.py", line 47, in activate code = compile(f, activate, 'exec') File "/Users/xxx/Library/Caches/pypoetry/virtualenvs/xxx-zz-py3.8/bin/activate_this.py", line 18 raise AssertionError(msg) from exc ^ SyntaxError: invalid syntax