patchcheck: use URL paths to identify upstream remote (GH-135806)

patchcheck: use URL paths to identify upstream remote (GH-135806)

* find defined "(fetch)" remotes with "python/cpython" in their URL
* if there is exactly one, use that remote name
* if there is one named "upstream", "origin", or "python",
  use that remote (in that precedence order)
* otherwise report an error listing the defined remotes