[3.11] gh-99032: datetime docs: Encoding is no longer relevant (GH-93365) (#103788)

This removes a section of the `strftime` and `strptime` documentation that refers to a bygone era when `strftime` would return an encoded byte string.

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(cherry picked from commit 2aa22f72fbbabb4ca2a641c0546d25c45128c56f)

Co-authored-by: William Andrea <william.j.andrea@gmail.com>
Co-authored-by: Paul Ganssle <1377457+pganssle@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>