Add ne validator

[jamesbeith]

Summary

This change adds a validator that raises ValueError if the
initializer is called with a number equal to val.

An example use case. I want a field that is a signed int of any
value except for zero. I could do the following.

attrs.field(
    validator=attrs.validators.or_(
        attrs.validators.le(-1),
        attrs.validators.ge(1),
    )
)

With this proposed ne validator I could do.

attrs.field(validator=attrs.validators.ne(0))

Pull Request Check List

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• There's tests for all new and changed code.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file typing-examples/baseline.py or, if necessary, typing-examples/mypy.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
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  • New functions/classes have to be added to docs/api.rst by hand.
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    The next version is the second number in the current release + 1.
    The first number represents the current year.
    So if the current version on PyPI is 26.2.0, the next version is gonna be 26.3.0.
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• Changes have news fragments in changelog.d.