Avoid generating generic and error types for attributes with other valid types

[tefra]

The type object and str shouldn't be generated

    <xs:element name="Example">
        <xs:complexType>
            <xs:sequence>
                <xs:element name="x" type="xs:anyType" maxOccurs="unbounded">
                    <xs:alternative test="@kind = 'quantity'">
                        <xs:complexType>
                            <xs:simpleContent>
                                <xs:extension base="xs:int">
                                    <xs:attribute ref="kind" use="required"/>
                                </xs:extension>
                            </xs:simpleContent>
                        </xs:complexType>
                    </xs:alternative>
                    <xs:alternative test="@kind = 'price'">
                        <xs:complexType>
                            <xs:simpleContent>
                                <xs:extension base="xs:double">
                                    <xs:attribute ref="kind" use="required"/>
                                </xs:extension>
                            </xs:simpleContent>
                        </xs:complexType>
                    </xs:alternative>
                    <xs:alternative test="@kind = 'mesg'">
                        <xs:complexType>
                            <xs:simpleContent>
                                <xs:extension base="xs:string">
                                    <xs:attribute ref="kind" use="required"/>
                                </xs:extension>
                            </xs:simpleContent>
                        </xs:complexType>
                    </xs:alternative>
                    <xs:alternative type="xs:error"/>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>

 x: List[Union[object, str, "Example.KindQuantity", "Example.KindPrice", "Example.KindMesg"]] = field(
        default_factory=list,
        metadata=dict(
            type="Element",
            namespace="",
            min_occurs=1,
            max_occurs=9223372036854775807
        )
    )