X golang_str: Fix bstr/ustr __add__ and friends to return NotImplemented wrt unsupported types

In bbbb58f0 (golang_str: bstr/ustr support for + and *) I've added
support for binary string operations, but similarly to __eq__ did not
handle correctly the case for arbitrary arguments that potentially
define __radd__ and similar.

As the result it breaks when running e.g. bstr + pyparsing.Regex

      File ".../pyparsing-2.4.7-py2.7.egg/pyparsing.py", line 6591, in pyparsing_common
        _full_ipv6_address = (_ipv6_part + (':' + _ipv6_part) * 7).setName("full IPv6 address")
      File "golang/_golang_str.pyx", line 469, in golang._golang._pybstr.__add__
        return pyb(zbytes.__add__(a, _pyb_coerce(b)))
      File "golang/_golang_str.pyx", line 243, in golang._golang._pyb_coerce
        raise TypeError("b: coerce: invalid type %s" % type(x))
    TypeError: b: coerce: invalid type <class 'pyparsing.Regex'>

because pyparsing.Regex is a type, that does not inherit from str, but defines
its own __radd__ to handle str + Regex as Regex.

-> Fix it by returning NotImplemented from under __add__ and other operations
where it is needed so that bstr and ustr behave in the same way as builtin str
wrt third types, but care to handle bstr/ustr promise that

    only explicit conversion through `b` and `u` accept objects with buffer interface. Automatic coercion does not.