fix possible KeyError

Author: picnixz

Lib/test/test_webbrowser.py

@@ -326,7 +326,8 @@ def setUp(self):
         # Ensure that 'BROWSER' is not set to 'open' or something else.
         # See: https://github.com/python/cpython/issues/131254.
         env = self.enterContext(os_helper.EnvironmentVarGuard())
-        env.unset("BROWSER")
+        if "BROWSER" in env:
+            env.unset("BROWSER")
 
         support.patch(self, os, "popen", self.mock_popen)
         self.browser = webbrowser.MacOSXOSAScript("default")