X golang_str: Fix bstr/ustr __eq__ and friends to return NotImplemented wrt non-string types

In 54c2a3cf (golang_str: Teach bstr/ustr to compare wrt any
string with automatic coercion) I've added __eq__, __ne__, __lt__ etc
methods to our strings, but __lt__ and other comparison to raise
TypeError against any non-string type. My idea was to mimic user-visible
py3 behaviour such as

    >>> "abc" > 1
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: '>' not supported between instances of 'str' and 'int'

However it turned out that the implementation was not exactly matching
what Python is doing internally which lead to incorrect behaviour when
bstr or ustr is compared wrt another type with its own __cmp__. In the
general case for `a op b` Python first queries a.__op__(b) and
b.__op'__(a) and sometimes other methods before going to .__cmp__. This
relies on the methods to return NotImplemented instead of raising an
exception and if a trial raises TypeError everything is stopped and that
TypeError is returned to the caller.

Jérome reports a real breakage due to this when bstr is compared wrt
distutils.version.LooseVersion . LooseVersion is basically

    class LooseVersion(Version):
        def __cmp__ (self, other):
            if isinstance(other, StringType):
                other = LooseVersion(other)

            return cmp(self.version, other.version)

but due to my thinko on `LooseVersion < bstr` the control flow was not
getting into that LooseVersion.__cmp__ because bstr.__gt__ was tried
first and raised TypeError.

-> Fix all comparison operations to return NotImplemented instead of
raising TypeError and make sure in the tests that this behaviour exactly
matches what native str type does.

The fix is needed not only for py2 because added test_strings_cmp_wrt_distutils_LooseVersion
was failing on py3 as well without the fix.

/reported-by @jerome
/reported-on nexedi/slapos!1575 (comment 206080)