gh-138775: fix handle `python -m base64` stdin correct with EOF signal (GH-138776)

* fix: handle  stdin correct with EOF single.
* fix: flollow the comments when pipe stdin use buffer
* Apply suggestions from code review
* fix: apply review comments in Lib/base64.py
* fix: address comments
* Reword comment and NEWS entry.

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Signed-off-by: yihong0618 <zouzou0208@gmail.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>