Add changelog entry

news/6536.feature

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+Add a beta version of pip's next-generation dependency resolver.
+
+Move pip's new resolver into beta, remove the
+``--unstable-feature=resolver`` flag, and enable the
+``--use-feature=2020-resolver`` flag. The new resolver is
+significantly stricter and more consistent when it receives
+incompatible instructions, and reduces support for certain kinds of
+:ref:`Constraints Files`, so some workarounds and workflows may
+break. More details about how to test and migrate, and how to report
+issues, at :ref:`Resolver changes 2020` . Maintainers are preparing to
+release pip 20.3, with the new resolver on by default, in October.