patch 8.2.0068: crash when using Python 3 with "utf32" encoding

Problem:    Crash when using Python 3 with "utf32" encoding. (Dominique Pelle)
Solution:   Use "utf-8" whenever enc_utf8 is set. (closes #5423)
diff --git a/src/testdir/test_python3.vim b/src/testdir/test_python3.vim
index 2853302..397299e 100644
--- a/src/testdir/test_python3.vim
+++ b/src/testdir/test_python3.vim
@@ -167,3 +167,10 @@
     call assert_match( '^Vim(.*):RuntimeError: TEST$', v:exception )
   endtry
 endfunc
+
+func Test_unicode()
+  " this crashed Vim once
+  set encoding=utf32
+  py3 print('hello')
+  set encoding=utf8
+endfunc