bpo-47042: Fix testing the HTML output in test_pydoc (GH-31959)

bpo-47042: Fix testing the HTML output in test_pydoc (GH-31959)

Previously it tested that that the actual output contains every non-whitespace
character from the expected output (ignoring order and repetitions).

Now it will test that the actual output contains the same lines as the expected
output, in the same order, ignoring indentation and empty lines.