MAINT: set minimum Sphinx version to 1.2.3, remove linkcode extension.

linkcode has been included in Sphinx 1.2, and has been giving a
warning when running the numpydoc tests for a long time.

We test with Sphinx 1.2.3 and latest; no need to keep supporting
even older versions than 1.2.3

Author: Ralf Gommers

numpydoc/linkcode.py

@@ -37,7 +37,7 @@
     author_email="[email protected]",
     url="https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt",
     license="BSD",
-    install_requires=["sphinx >= 1.0.1", 'Jinja2>=2.3'],
+    install_requires=["sphinx >= 1.2.3", 'Jinja2>=2.3'],
     package_data={'numpydoc': ['tests/test_*.py', 'templates/*.rst']},
     test_suite = 'nose.collector',
     cmdclass={"sdist": sdist},