3.14.0a1 urljoin regression wrt //

Bug report

Bug description:

The tests of lxml do (simplified):

from urllib.parse import urljoin
urljoin('file:', '/foo/bar')

• On Python 3.13.0 and earlier, the result of the urljoin call is file:///foo/bar.
• On Python 3.14.0a1, the result of the urljoin call is file:/foo/bar.

This change of behavior was introduced in fc897fcc01964649f023e0baa4c95d142e4e8a10 https://github.com/python/cpython/pull/123273 cc @serhiy-storchaka

The NEWS entry in that change does not seem to indicate this change was intentional.

CPython versions tested on:

3.14, CPython main branch

Operating systems tested on:

Linux

从 urllib.parse.urljoin 开始，检查 fc897fcc01964649f023e0baa4c95d142e4e8a10 引入的行为变化。在 Python 3.14 上复现 lxml 风格的示例，并确认完成的修复会为 urljoin('file:', '/foo/bar') 恢复 file:///foo/bar，同时不会使报告的变化发生回归。