Inconsistent use of `attr` parameter and `__eq__` in VersionConstraint

[keshav-space]

For __eq__ method to take effect, eq=False must be added as a attr parameter
https://github.com/nexB/univers/blob/7206c1fc35eb32d0006591e6defcc65ded40cfa0/src/univers/version_constraint.py#L123-L127
Something like this:

@attr.s(frozen=True, repr=True, str=False, order=False, eq=False, hash=True) 

Current attr paramaters:
https://github.com/nexB/univers/blob/7206c1fc35eb32d0006591e6defcc65ded40cfa0/src/univers/version_constraint.py#L55-L56