make runtox.py not import external modules unless run as a script

Was not needed since the script actually does nothing unless run as a script.

--HG--
branch : jurko/tox_usage

runtox.py

@@ -1,9 +1,8 @@
 #!/usr/bin/env python
 
-import subprocess
-import sys
-
 if __name__ == "__main__":
+    import subprocess
+    import sys
     subprocess.call([sys.executable, "-m", "tox",
                      "-i", "ALL=https://devpi.net/hpk/dev/",
                      "--develop"] + sys.argv[1:])