GH-125866: RFC8089 file URIs in urllib.request

[barneygale]

Adjust urllib.request.pathname2url() and url2pathname() to generate and accept file URIs as described in RFC8089.

pathname2url() gains a new include_scheme argument, which defaults to false. When set to true, the returned URL includes a file: prefix. url2pathname() now automatically removes a file: prefix if present.

On Windows, pathname2url() now generates URIs that begin with two slashes rather than four when given a UNC path.

On other platforms, pathname2url() now generates URIs that begin with three slashes rather than one when given an absolute path. url2pathname() now performs the opposite transformation, so file:///etc/hosts becomes /etc/hosts. Furthermore, url2pathname() now ignores local hosts (like "localhost" or any alias) and raises URLError for non-local hosts.

pathname2url() examples (star marks differences):

	previously	now
pathname2url('foo/bar')	foo/bar	foo/bar
pathname2url('/foo/bar')	/foo/bar	///foo/bar *
pathname2url('//foo/bar') (POSIX)	//foo/bar	////foo/bar *
pathname2url('//foo/bar') (Windows)	////foo/bar	//foo/bar *

url2pathname() examples (star marks differences):

	previously	now
url2pathname('foo/bar')	foo/bar	foo/bar
url2pathname('/foo/bar')	/foo/bar	/foo/bar
url2pathname('//foo/bar') (POSIX)	//foo/bar	raise URLError *
url2pathname('//foo/bar') (Windows)	//foo/bar	//foo/bar
url2pathname('///foo/bar')	///foo/bar	/foo/bar *
url2pathname('////foo/bar') (POSIX)	////foo/bar	//foo/bar *
url2pathname('////foo/bar') (Windows)	//foo/bar	//foo/bar
url2pathname('/////foo/bar') (Windows)	///foo/bar	//foo/bar *

• Issue: Improve file URI ergonomics in urllib.request #125866

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[barneygale]

Test failures will go away when #125739 lands