bpo-42369: Fix thread safety of zipfile._SharedFile.tell (GH-26974)

bpo-42369: Fix thread safety of zipfile._SharedFile.tell (GH-26974)

The `_SharedFile` tracks its own virtual position into the file as
`self._pos` and updates it after reading or seeking. `tell()` should
return this position instead of calling into the underlying file object,
since if multiple `_SharedFile` instances are being used concurrently on
the same file, another one may have moved the real file position.
Additionally, calling into the underlying `tell` may expose thread
safety issues in the underlying file object because it was called
without taking the lock.
(cherry picked from commit e730ae7effe4f13b24f1b5fb1fca005709c86acb)

Co-authored-by: Kevin Mehall <km@kevinmehall.net>