gh-155742: Remove dead code in _elementtree

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Add tests on TreeBuilder.data().

• Issue: Treat bytes objects as immutable in C code #155742

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@serhiy-storchaka: Should we modify data() to raise TypeError if the argument is not a string?

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Calling TreeBuilder.data() with a bytes object fails on TreeBuilder.close() call with the Python implementation, but it doesn't fail with the C implementation.

Calling TreeBuilder.data() twice with a bytes object fails on TreeBuilder.close() with the Python implementation, but it only fails later when reading the .text attribute after calling TreeBuilder.close() with C implementation.

TreeBuilder.data() documentation says:

Adds text to the current element. data is a string.

But it doesn't raise a TypeError if the argument is not a str instance or subclass.

Modifying data() to raise a TypeError in Python and C implementations would make the two implementations behave the same, but I'm a worried that it can break an unusual use case relaying on the current behavior of the C (default) implementation.

An alternative is to only remove the PyBytes_CheckExact() C code and leaves the test suite unchanged :-)

[serhiy-storchaka]

This is a part of Python 2 leftover. I created a separate issue for this (#157366) and a more complete PR.

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@serhiy-storchaka wrote a more complete PR than mine: PR gh-157367. I close my PR.