patch 8.2.0578: heredoc for interfaces does not support "trim"

Problem:    Heredoc for interfaces does not support "trim".
Solution:   Update the script heredoc support to be same as the :let command.
            (Yegappan Lakshmanan, closes #5916)

src/testdir/test_python2.vim

@@ -167,3 +167,22 @@ func Test_Catch_Exception_Message()
     call assert_match( '^Vim(.*):RuntimeError: TEST$', v:exception )
   endtry
 endfunc
+
+" Test for various heredoc syntax
+func Test_python_heredoc()
+  python << END
+s='A'
+END
+  python <<
+s+='B'
+.
+  python << trim END
+    s+='C'
+  END
+  python << trim
+    s+='D'
+  .
+  call assert_equal('ABCD', pyxeval('s'))
+endfunc
+
+" vim: shiftwidth=2 sts=2 expandtab