BUG: numpydoc/linkcode: do not detect linkcode config changes

Since the linkcode_resolve is a function, it triggers configuration
change on each rebuild, resulting to full rebuild always. It's better
to make it not do that.
DOC: numpydoc/linkcode: mention that the extension will be in Sphinx upstream
ENH: numpydoc: deal with duplicated signatures
ENH: numpydoc: Python 2 & 3 in single codebase, restructure as a package
MAINT: always use plot directive from Matplotlib, and prefer Sphinx linkcode
TST: numpydoc: add stub test files, to check that files at least import
BUG: numpydoc: fix bugs in attribute docstring extraction + improve presentation
TST: numpydoc: more class tests
BUG: numpydoc: check that it works with sub-classes
DOC: fix doc/source/conf.py to work with Python 3
Merge branch 'enh-numpydoc'

There were some conflicts with the 2to3 work in numpy. I think I got the
fixes right.

* enh-numpydoc:
  DOC: fix doc/source/conf.py to work with Python 3
  BUG: numpydoc: check that it works with sub-classes
  TST: numpydoc: more class tests
  BUG: numpydoc: fix bugs in attribute docstring extraction + improve presentation
  TST: numpydoc: add stub test files, to check that files at least import
  MAINT: always use plot directive from Matplotlib, and prefer Sphinx linkcode
  ENH: numpydoc: Python 2 & 3 in single codebase, restructure as a package
  ENH: numpydoc: deal with duplicated signatures
  DOC: numpydoc/linkcode: mention that the extension will be in Sphinx upstream
  BUG: numpydoc/linkcode: do not detect linkcode config changes

Conflicts:
	doc/sphinxext/numpydoc/docscrape.py
	doc/sphinxext/numpydoc/docscrape_sphinx.py
	doc/sphinxext/numpydoc/linkcode.py
	doc/sphinxext/numpydoc/phantom_import.py
	doc/sphinxext/numpydoc/traitsdoc.py