Add `seekable` method for `mmap.mmap`

Bug report

Bug description:

mmap.mmap has a seek() method, but no seekable() method. Because of this, it cannot be used an argument for ZipFile, which requires a file-like object with seekable. A small patch is enough to fix this

class SeekableMmap(mmap.mmap):
    def seekable(self):
        return True

Now SeekableMmap can be passed to ZipFile without issue. It's easy to fix, but I can't think of any reasons why mmap shouldn't have a seekable method.

CPython versions tested on:

3.8, 3.11

Operating systems tested on:

Linux

Linked PRs

• gh-111835: Add seekable method to mmap.mmap #111852
• pythongh-111835: Add seekable method to mmap.mmap #111865