GeezerVison

@serhiy-storchaka cc @vstinner
As the issue reporter said, they can use seekable by subclassing mmap itself.
But this change definitely helpful to people who need to use library which checks that the file object supports seekable() method.
(I don't want to say it must be added but it is good to be added)

mmap.seek() always return None. It can make mmap not appropriate substitution for files. Adding seekable() method can break existing code.

mmap.seek() always return None. It can make mmap not appropriate substitution for files. Adding seekable() method can break existing code.

I left a comment for this.
#111835 (comment)
But I'm not sure how it will break the user's code even if we update seek() to return the current position.
I can not guess how people are doing something with None value.

@serhiy-storchaka @vstinner
Thank you for the reviews!!