[3.12] gh-80109: Fix io.TextIOWrapper dropping the internal buffer during write() (GH-22535) (GH-113808)

io.TextIOWrapper was dropping the internal decoding buffer
during read() and write() calls.
(cherry picked from commit 73c93265634257b1488262097e024c1727260cfd)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>