GH-119496: accept UTF-8 BOM in .pth files (GH-119503)

`Out-File -Encoding utf8` and similar commands in Windows Powershell 5.1 emit
UTF-8 with a BOM marker, which the regular `utf-8` codec decodes incorrectly.

`utf-8-sig` accepts a BOM, but also works correctly without one.

This change also makes .pth files match the way Python source files are handled.

Co-authored-by: Inada Naoki <songofacandy@gmail.com>