patch 8.2.0075: Python 3 unicode test still sometimes fails

Problem:    Python 3 unicode test still sometimes fails.
Solution:   Skip the test when 'termencoding' is not empty.
diff --git a/src/testdir/test_python3.vim b/src/testdir/test_python3.vim
index cd5ac9c..f722edc 100644
--- a/src/testdir/test_python3.vim
+++ b/src/testdir/test_python3.vim
@@ -170,8 +170,9 @@
 
 func Test_unicode()
   " this crashed Vim once
-  let save_tenc = &tenc
-  set tenc=
+  if &tenc != ''
+    throw "Skipped: 'termencoding' is not empty"
+  endif
 
   set encoding=utf32
   py3 print('hello')
@@ -183,5 +184,4 @@
   py3 print('hello')
 
   set encoding=utf8
-  let &tenc = save_tenc
 endfunc