Issue #12062: Fix a flushing bug when doing a certain type of I/O sequence

on a file opened in read+write mode (namely: reading, seeking a bit forward,
writing, then seeking before the previous write but still within buffered
data, and writing again).

diff --cc Lib/test/test_io.py
Simple merge

diff --cc Misc/NEWS
Simple merge

diff --cc Modules/_io/bufferedio.c
Simple merge