Fixed https://github.com/amix/vimrc/issues/546

This is done by removing custom FileType indent options, and using
`vim-python-pep8-indent` plugin.

sources_non_forked/vim-python-pep8-indent/CONTRIBUTING.rst

@@ -0,0 +1,37 @@
+How To Contribute
+=================
+
+``vim-python-pep8-indent`` is always open for suggestions and contributions by generous developers.
+I’ve collected a few tips to get you started.
+
+Please:
+
+- *Always* add tests for your code.
+- Write `good commit messages`_.
+
+
+Running Tests
+-------------
+
+- They are written in Ruby_ (sorry :() using vimrunner_ which requires rspec_.
+- The tests go into ``spec/indent/indent_spec.rb``.
+  Look at the ``describe`` blocks to get the hang of it.
+- Run the tests with the command::
+
+   $ rspec spec
+- Alternatively you can use Docker::
+
+   $ make test_docker
+
+- You can select tests based on line numbers, e.g.::
+
+   $ rspec ./spec/indent/indent_spec.rb:385
+   $ make test_docker RSPEC_ARGS=./spec/indent/indent_spec.rb:385
+
+Thank you for considering to contribute!
+
+
+.. _Ruby: https://www.ruby-lang.org/
+.. _`good commit messages`: http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
+.. _vimrunner: https://github.com/AndrewRadev/vimrunner
+.. _rspec: https://github.com/rspec/rspec