Enable doctests in pydoctor docstrings

[tristanlatr]

We currently use doctests in the testing but only through .doctest files under pydoctor/test/epydoc/.
Some objects like ParsedTypeDocstring, TypeDocstring, modify_iter, _configparser.py have doctests that demonstrate the usage or implementation details. So enabling this feature should help maintaining this documentation in the time.

We also have case like the in the astutils.iterassign where the docstest is not used to demontrate code usage but simply to colorize the python code. These can be replaced by epytext literal block syntax ::.