pydoc: extract/expand module path in python code

s:ExpandModulePath is taken from @fs111's pydoc.vim:
https://github.com/fs111/pydoc.vim/blob/master/ftplugin/python_pydoc.vim
Merge pull request #37 from blueyed/pydoc-expandmodulepath

pydoc: extract/expand module path in python code