stack/slapos.cfg: fix eggs version for python3.7

Werkzeug 3.0.0 is not compatible with Python3.7:

  File "/opt/slapos/eggs/werkzeug-3.0.0-py3.7.egg/werkzeug/serving.py",
line 76, in <module>
    t.Union["ssl.SSLContext", t.Tuple[str, t.Optional[str]],
t.Literal["adhoc"]]
AttributeError: module 'typing' has no attribute 'Literal