[3.13] gh-148192: Fix Generator._make_boundary behavior with CRLF line endings. (GH-148193) (#148549)

The Generator._make_boundary regex did not match on boundary phrases correctly when using CRLF line endings due to re.MULTILINE not considering \r\n as a line ending.
(cherry picked from commit 4af46b4ab5af49d8df034320a9a70fcbb062f7cf)

Co-authored-by: Henry Jones <44321887+henryivesjones@users.noreply.github.com>