bpo-36502: Update link to UAX python#44, the Unicode doc on the UCD.

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.

Doc/library/unicodedata.rst

@@ -22,7 +22,7 @@ this database is compiled from the `UCD version 12.1.0
 
 The module uses the same names and symbols as defined by Unicode
 Standard Annex #44, `"Unicode Character Database"
-<http://www.unicode.org/reports/tr44/tr44-6.html>`_.  It defines the
+<http://www.unicode.org/reports/tr44/>`_.  It defines the
 following functions: