Fix name clash caused by shadowed filter().

We shouldn't waste time formatting code that is really an external
dependency, that we are just bundling for convenience.

Import moved modules from theano.compat.six.moves.
Use six.moves.StringIO rather than already available io.StringIO.

Also remove unused import of Callable.

Not altering it to pass flake8 since this is external code, we should
just whitelist it. Better yet, we should add it as a dependency and
remove the bundled version.

Do not try to pickle `method_descriptor` objects

improving debugprint for scan ops