FileIO.seekable() is incorrect if determined by an invalid seek call #159677

Bug report

While working on concurrency fixes for FileIO, I discovered an edge case (quite rare...) that makes seekable incorrect if determined by a call to seek that results in an error.

Reproducer

f = io.FileIO(tempfile.mktemp(), 'w+')
f.seek(-1)       # OSError: [Errno 22] Invalid argument
f.seekable()     # False but it should be true

The problem comes from this part of the implementation for seek, which sets seekable regardless of what the problem was (which could be that the file indeed is not seekable, or that in our example the passed offset was not valid).

This PR should fix it: #159678

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-159677: fix FileIO.seekable incorrectly returning false #159678