Fix tests on PyPy3.

Previously they would fail like this:

  File /home/travis/build/zopefoundation/zope.schema/src/zope/schema/_bootstrapfields.py, line 522, in fromUnicode
    raise self._validation_error(*last_exc.args).with_field_and_value(self, value)
TypeError: __init__() missing 1 required positional argument: 'literal'

src/zope/schema/_field.py

@@ -257,10 +257,7 @@ class Float(Real):
 
 
 class InvalidDecimalLiteral(ValueError, ValidationError):
-
-    def __init__(self, literal):
-        super(InvalidDecimalLiteral, self).__init__(
-            "invalid literal for Decimal(): %s" % literal)
+    "Raised by decimal fields"
 
 
 @implementer(IDecimal, IFromUnicode)