Deprecate raising unittest.SkipTest to skip tests during collection

It is not very clear why this code exists -- we are not running any
unittest or nose code during collection, and really these frameworks
don't have the concept of collection at all, and just raising these
exceptions at e.g. the module level would cause an error. So unless I'm
missing something, I don't think anyone is using this.

Deprecate it so we can eventually clear up this code and keep unittest
more tightly restricted to its plugin.

changelog/8242.deprecation.rst

@@ -0,0 +1,7 @@
+Raising :class:`unittest.SkipTest` to skip collection of tests during the
+pytest collection phase is deprecated. Use :func:`pytest.skip` instead.
+
+Note: This deprecation only relates to using `unittest.SkipTest` during test
+collection. You are probably not doing that. Ordinary usage of
+:class:`unittest.SkipTest` / :meth:`unittest.TestCase.skipTest` /
+:func:`unittest.skip` in unittest test cases is fully supported.