gh-161271: Drop the read buffer in _pyio.BufferedRandom.truncate() - #161273

_io.BufferedRandom.truncate() rewinds the raw stream and resets the read buffer
before truncating (buffered_flush_and_rewind_unlocked() in
Modules/_io/bufferedio.c). _pyio does not, although the comment there already
says "Use seek to flush the read buffer", so a read after truncate() can return
data that is no longer in the file:

>>> import _pyio, os
>>> open("x", "wb").write(b"abcdef")
6
>>> f = _pyio.open("x", "r+b"); f.read(1); f.truncate(0); f.read(), os.path.getsize("x")
(b'bcdef', 0)

The one-line self.seek(self.tell()) keeps the logical position unchanged (as
truncate() must) while rewinding the raw stream and emptying the read buffer, which
is what _io does.

Tested: the reproducer and the new test were run against _io, against unpatched
_pyio and against patched _pyio on 3.13 (the touched code is byte-identical on
main), plus a 25-case differential battery over r+b/ab+/w+b and truncation
points 0/1/3/None comparing return value, tell(), following read() and the
resulting file size: 12 divergences from _io before, 0 after. I have not run the
full test_io suite locally (no local build yet), so CI is the check for that.

• Issue: _pyio.BufferedRandom.truncate() keeps the read buffer, so later reads return data that is no longer in the file #161271