verify: skip tkinter test if TCL_LIBRARY not set

This is a bit brittle. But it is an easy way to handle the case
where tcl/tk aren't present, such as the case with Windows static
distributions.

Author: indygreg

src/verify_distribution.py

@@ -121,6 +121,7 @@ def test_ssl(self):
 
         ssl.create_default_context()
 
+    @unittest.skipIf("TCL_LIBRARY" not in os.environ, "TCL_LIBRARY not set")
     @unittest.skipIf("DISPLAY" not in os.environ, "DISPLAY not set")
     def test_tkinter(self):
         import tkinter as tk