python27Packages.pytestpep8: remove due to pytest v6 incompatibility

Remove ``pythonPackages.pytestpep8`` because:
* It has been abandoned upstream: no PyPi release since 2014
* It is no longer compatible with pytest v6+: I get an error pointing to
  https://docs.pytest.org/en/stable/deprecations.html#node-construction-changed-to-node-from-parent
  when I try to use it with ``pytestCheckHook`` (i.e. pytest v6.0+)
* It doesn't have a maintainer

pkgs/top-level/python-packages.nix

@@ -5803,7 +5803,7 @@ in {
 
   pytest-pep257 = callPackage ../development/python-modules/pytest-pep257 { };
 
-  pytestpep8 = callPackage ../development/python-modules/pytest-pep8 { };
+  pytestpep8 = throw "pytestpep8 was removed because it is abandoned and no longer compatible with pytest v6.0"; # added 2020-12-10
 
   pytest-pylint = callPackage ../development/python-modules/pytest-pylint { };