Complete error signature of _InValidator (#951)

The docstring of _in() says the ValueError would contain these, however
it didn't. Adding said arguments enables uniform processing of
ValueErrors raised by validators. One might for example want to extract
the attr.name and the value that raised the error to show a custom, more
user-friendly error message.

changelog.d/951.change.rst

@@ -0,0 +1,3 @@
+The error signature of _InValidator has been completed,
+it now includes the attribute,
+options and value as proclaimed in the docstring of _in().

src/attr/validators.py

@@ -299,7 +299,10 @@ def __call__(self, inst, attr, value):
             raise ValueError(
                 "'{name}' must be in {options!r} (got {value!r})".format(
                     name=attr.name, options=self.options, value=value
-                )
+                ),
+                attr,
+                self.options,
+                value,
             )
 
     def __repr__(self):

tests/test_validators.py

@@ -471,7 +471,12 @@ def test_fail(self):
         a = simple_attr("test")
         with pytest.raises(ValueError) as e:
             v(None, a, None)
-        assert ("'test' must be in [1, 2, 3] (got None)",) == e.value.args
+        assert (
+            "'test' must be in [1, 2, 3] (got None)",
+            a,
+            [1, 2, 3],
+            None,
+        ) == e.value.args
 
     def test_fail_with_string(self):
         """
@@ -482,7 +487,12 @@ def test_fail_with_string(self):
         a = simple_attr("test")
         with pytest.raises(ValueError) as e:
             v(None, a, None)
-        assert ("'test' must be in 'abc' (got None)",) == e.value.args
+        assert (
+            "'test' must be in 'abc' (got None)",
+            a,
+            "abc",
+            None,
+        ) == e.value.args
 
     def test_repr(self):
         """