[3.13] gh-52551: Fix encoding issues in strftime() (GH-125193) (GH-125657)

[3.13] gh-52551: Fix encoding issues in strftime() (GH-125193) (GH-125657)

Fix time.strftime(), the strftime() method and formatting of the
datetime classes datetime, date and time.

* Characters not encodable in the current locale are now acceptable in
  the format string.
* Surrogate pairs and sequence of surrogatescape-encoded bytes are no
  longer recombinated.
* Embedded null character no longer terminates the format string.

This fixes also gh-78662 and gh-124531.

(cherry picked from commit ad3eac1963a5f195ef9b2c1dbb5e44fa3cce4c72)