patch 8.2.4396: Python3 test fails

Problem:    Python3 test fails.
Solution:   Remove "let".
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 65d6dc8..218b252 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -3766,7 +3766,7 @@
 if has('python3')
   def Test_python3_command()
     py3 import vim
-    py3 vim.command("let g:done = 'yes'")
+    py3 vim.command("g:done = 'yes'")
     assert_equal('yes', g:done)
     unlet g:done
   enddef