setup.py: Switch to setuptools

setup.py: Switch to setuptools

This removes a lot of distutils overrides we were using in favour of
good old-fashioned 'MANIFEST.in' entries. This appears to generate the
almost the exact same artifacts with the exception of a single new file,
'docutils/writers/latex2e/docutils-05-compat.sty'. However, this file is
referenced from multiple documents, including the LaTeX writer user
guide ('docs/user/latex.txt') and a document describing the file itself
('docs/user/docutils-05-compat.sty.txt'), suggesting this was a mistake
that we're correcting here.

This comparison was made as follows.

1. Create directories to backup artifacts.

     $ mkdir -p /tmp/docutils/old
     $ mkdir -p /tmp/docutils/new

2. Using the code before this change, build a package and copy it to the
   backup directory.

     $ virtualenv .venv --python python3.6
     $ source .venv/bin/activate
     $ cd docutils
     $ python setup.py build
     $ mv build /tmp/docutils/old/
     $ cd -
     $ deactivate
     $ rm -rf .venv

3. Repeat step 2 but with this change applied, using '/tmp/docutils/new'
   as the backup location.

4. Compare the directories.

     $ cd /tmp/docutils
     $ diff -burq new/build/ old/build/
     Only in new/build/lib/docutils/writers/latex2e: docutils-05-compat.sty

5. Repeat steps 2-4 but using the 'python setup.py bdist_wheel' command,
   copying the 'dist' directory (rather than the 'build' directory), and
   extracting generated wheel using the 'unzip' command.

Signed-off-by: Stephen Finucane <stephen@that.guru>
git-svn-id: http://svn.code.sf.net/p/docutils/code/trunk@8484 929543f6-e4f2-0310-98a6-ba3bd3dd1d04