get package version for docs

so we don't need to manually update version strings

docs/source/conf.py

@@ -91,10 +91,13 @@ author = 'The Jupyter Team'
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
+_version_py = '../../notebook/_version.py'
+version_ns = {}
+exec(compile(open(_version_py).read(), _version_py, 'exec'), version_ns)
 # The short X.Y version.
-version = '4.1'
+version = '%i.%i' % version_ns['version_info'][:2]
 # The full version, including alpha/beta/rc tags.
-release = '4.1.0rc1'
+release = version_ns['__version__']
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.