VersionRange objects are unhashable

[Hritik14]

Is this by design ? It doesn't seem like it because of hash=True in https://github.com/nexB/univers/blob/a52ea9900d87ef6cd0ed989bf20a41a794cf39c4/src/univers/version_range.py#L18

>>> vr = NginxVersionRange.from_string("vers:nginx/3")
>>> X = set()
>>> X.add(vr)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-25340b8f0bdf> in <module>
----> 1 X.add(vr)

<attrs generated hash univers.version_range.VersionRange> in __hash__(self)
      1 def __hash__(self):
----> 2         return hash((
      3                 938370967647186531,
      4                 self.constraints,
      5             ))
TypeError: unhashable type: 'list'