Improve file URI ergonomics in `urllib.request`

[barneygale]

Feature or enhancement

I request that we make pathname2url and url2pathname easier to use:

• pathname2url() is made to accept an optional include_scheme argument that sticks file: on the front when true
• url2pathname() is made to strip any file: prefix from its argument.

I think this would go a long way towards making these functions usable, and allow us to remove the scary "This does not accept/produce a complete URL" warnings from the docs.

Linked PRs

• GH-125866: Improve tests for pathname2url() and url2pathname() #125993
• [3.13] GH-125866: Improve tests for pathname2url() and url2pathname() (GH-125993) #126144
• [3.12] GH-125866: Improve tests for pathname2url() and url2pathname() (GH-125993) #126145
• GH-125866: Preserve Windows drive letter case in file URIs #127138
• GH-125866: Deprecate nturl2path module #131432
• GH-125866: Support complete "file:" URLs in urllib #132378