Comments on the patch:
- you should check the error return of lseek() (and possibly wrap it in
Py_BEGIN/END_ALLOW_THREADS, see portable_lseek() in the same file)
- there should be a test for each of unbuffered IO (buffering=0),
buffered IO ("rb") and text IO ("r"). For text IO, the test shouldn't
test the actual value returned by tell(), only that it is > 0 (because
tell() in text mode is an opaque value and is not necessarily equal to a
byte position)