Avoid switching python interpreter when publishing

If setup.py is run via `python2.7 setup.py publish`, the current code
will invoke the default python, rather than the explicitly called one.

Avoid that by spawning the same interpreter.

setup.py

@@ -27,7 +27,7 @@ with open(os.path.join(here, NAME, "__version__.py")) as f:
 
 # Support "$ setup.py publish".
 if sys.argv[-1] == "publish":
-    os.system("python setup.py sdist bdist_wheel upload")
+    os.system("{} setup.py sdist bdist_wheel upload".format(sys.executable))
     sys.exit()
 
 # What packages are required for this module to be executed?