RFC8089-compliant file URI support in urllib.request

[barneygale]

Feature or enhancement

I request that we make pathname2url and url2pathname parse and generate rfc8089-compliant file URIs. In particular, this means:

• On Windows:
  • pathname2url() should return a URL with a non-empty authority when given a UNC path
    • e.g. pathname2url('//server/share') -> //server/share rather than ////server/share
  • url2pathname() should ignore a 'localhost' authority
    • e.g. url2pathname('//localhost/c:/foo') -> c:/foo
• On POSIX:
  • pathname2url() should return a URL with a zero-length authority when given an absolute path
    • e.g. pathname2url('/etc/hosts') -> ///etc/hosts rather than /etc/hosts
  • url2pathname() should ignore any empty or local authority
    • e.g. url2pathname('///etc/hosts') -> /etc/hosts rather than ///etc/hosts
    • e.g. url2pathname('//127.0.0.1/etc/hosts) -> /etc/hosts.

I further request that:

• 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.