Mention PEP 626 in the changelog

Author: nedbat

CHANGES.rst

@@ -29,10 +29,15 @@ Unreleased
   of the output easier.  Thanks, Judson Neer.  This had been requested a number
   of times (`issue 1086`_, `issue 922`_, `issue 732`_).
 
+- Update to support Python 3.10 alphas in progress, including `PEP 626: Precise
+  line numbers for debugging and other tools <pep626_>`_.
+
 .. _issue 1086: https://github.com/nedbat/coveragepy/issues/1086
 .. _issue 732: https://github.com/nedbat/coveragepy/issues/732
 .. _issue 922: https://github.com/nedbat/coveragepy/issues/922
 
+.. _pep626: https://www.python.org/dev/peps/pep-0626/
+
 
 .. _changes_531: