[3.13] gh-129843: fix pure Python implementation of `warnings.warn_explicit` (GH-129848) (#131349)

gh-129843: fix pure Python implementation of `warnings.warn_explicit` (GH-129848)

The pure Python implementation of `warnings.warn_explicit` constructs a `WarningMessage`
with an incorrect source (it incorrectly sets the WarningMessage's line to the given `source`).
(cherry picked from commit 80e00ecc399db8aeaa9f3a1c87a2cfb34517d7be)

Co-authored-by: Srinivas Reddy Thatiparthy (తాటిపర్తి శ్రీనివాస్  రెడ్డి) <thatiparthysreenivas@gmail.com>