patch 8.2.0672: heredoc in scripts does not accept lower case marker

Problem:    Heredoc in scripts does not accept lower case marker.
Solution:   Allow lower case only in non-Vim scripts. (Ken Takata,
            closes #6019)

src/testdir/test_python3.vim

@@ -349,7 +349,10 @@ s+='B'
   python3 << trim
     s+='D'
   .
-  call assert_equal('ABCD', pyxeval('s'))
+  python3 << trim eof
+    s+='E'
+  eof
+  call assert_equal('ABCDE', pyxeval('s'))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab