Accommodate for recent pathname2url() changes upstream

[ichard26]

• UNC paths converted to URLs now start with two slashes, like earlier (yes, really!)
• Trailing slashes are now preserved on Windows, matching POSIX behaviour

Fixes #13104.

[ichard26]

/cc @pfmoore @uranusjr for your Windows expertise.

[pfmoore]

Frankly, I think the way Windows pathnames are translated to URLs is a dumpster fire, and I wish we could avoid using it at all. But given that we have to, this is as good a way as any of testing it works for what we need.

[notatallshaw]

I think the way Windows pathnames are translated to URLs is a dumpster fire

I just recently saw this comment and certainly never realized before it was so confusing: #13062 (comment)

[pfmoore]

That's not even all of it. There's variants like file://C:|Windows/System32 which I believe are obsolete, but still found in the wild 🙁

[barneygale]

I am trying to extinguish the url2pathname() and pathname2url() dumpster fires, but there are a lot of wrongs to right! I believe there will be no more backports, with the remaining work being 3.14+ only.