tests: Don't require tkinter to be able to run test suite

tests/conftest.py

@@ -1,5 +1,4 @@
 import sys
-import tkinter as tk
 import pytest
 from panda3d import core
 from direct.showbase.ShowBase import ShowBase
@@ -14,6 +13,8 @@ def base():
 
 @pytest.fixture
 def tk_toplevel():
+    tk = pytest.importorskip('tkinter')
+
     if sys.platform == 'darwin' and not core.ConfigVariableBool('want-tk', False):
         pytest.skip('"want-tk" must be true to use tkinter with Panda3D on macOS')
     try: