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)
diff --git a/src/testdir/test_pyx3.vim b/src/testdir/test_pyx3.vim
index 5b983e4..5d38420 100644
--- a/src/testdir/test_pyx3.vim
+++ b/src/testdir/test_pyx3.vim
@@ -94,7 +94,10 @@
   pyx << trim
     result+='D'
   .
-  call assert_equal('ABCD', pyxeval('result'))
+  pyx << trim eof
+    result+='E'
+  eof
+  call assert_equal('ABCDE', pyxeval('result'))
 endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab