bpo-42988: Remove the pydoc getfile feature (GH-25015) (GH-25067)

CVE-2021-3426: Remove the "getfile" feature of the pydoc module which
could be abused to read arbitrary files on the disk (directory
traversal vulnerability). Moreover, even source code of Python
modules can contain sensitive data like passwords. Vulnerability
reported by David Schwörer.
(cherry picked from commit 9b999479c0022edfc9835a8a1f06e046f3881048)

Co-authored-by: Victor Stinner <vstinner@python.org>