bpo-36502: Update link to UAX #44, the Unicode doc on the UCD. (GH-15301)

The link we have points to the version from Unicode 6.0.0, dated 2010.
There have been numerous updates to it since then:
  https://www.unicode.org/reports/tr44/#Modifications

Change the link to one that points to the current version. Also, use HTTPS.