update determination of short version to work with PEP 440 style

doc/source/conf.py

@@ -52,12 +52,8 @@ copyright = u'2011-2015, C. W. Groth (CEA), M. Wimmer, A. R. Akhmerov, X. Wainta
 # The full version, including alpha/beta/rc tags.
 release = kwant.__version__
 
-for i, s in enumerate(release):
-    if s not in '0123456790.':
-        break
-
 # The short X.Y version.
-version = release[:i]
+version = release[:len(release) - len(release.lstrip('012345679.'))].rstrip('.')
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.