As far as I can see, Sphinx has a global setting for trim_doctest_flags but lacks the possibility of locally disabling the trimming.

A quick workaround would be to have the following sphinx extension added:

class ProxyLexer(object):

    def __init__(self, underlying):
        self.__underlying = underlying

    def __getattr__(self, attr):
        return getattr(self.__underlying, attr)

def setup(app):
    from sphinx.highlighting import lexers
    if lexers is not None:
        lexers['pycon-literal'] = ProxyLexer(lexers['pycon'])
        lexers['pycon3-literal'] = ProxyLexer(lexers['pycon3'])

That would allow blocks marked as

.. code-block:: pycon-literal

or preceded by 

.. highlight:: pycon-literal

to escape the trimming of doctest flags.

If that's of any interest I can submit a patch.