Update changelog and remove last cruft.

Author: ionelmc

CHANGELOG.rst

@@ -2,12 +2,25 @@ Changelog
 =========
 
 
-3.1.0 (future)
+4.0.0 (future)
 -------------------
 
+**Note that this release drops support for multiprocessing.**
+
+
 * `--cov-fail-under` no longer causes `pytest --collect-only` to fail
   Contributed by Zac Hatfield-Dodds in
   `#511 <https://github.com/pytest-dev/pytest-cov/pull/511>`_.
+* Dropped support for multiprocessing (mostly because `issue 82408 <https://github.com/python/cpython/issues/82408>`_). This feature was
+  mostly working but made out test suite very flaky and slow.
+
+  There is builtin multiprocessing support in coverage and you can switch to that if you feel lucky. All you need is this in your
+  ``.coveragerc``::
+
+    [run]
+    concurrency = multiprocessing
+    parallel = true
+    sigterm = true
 
 
 3.0.0 (2021-10-04)

tests/test_pytest_cov.py

@@ -1400,13 +1400,6 @@ def test_basic(no_cover):
     raise NotImplementedError
 '''
 
-MP_COVERAGERC = '''
-[run]
-concurrency = multiprocessing
-parallel = true
-sigterm = True
-'''
-
 EXCLUDED_TEST = '''
 
 def func():