Fix manifest

This adds a failing CI entry that checks the manifest.

Currently, it complains about the following files and we should make an explicit decision whether we want to ship them with our `sdist` or not. I have added some uncontroversial entries, which reduces the list to:

```
  admin
  admin/doctest-all.py
  admin/l10n-update
  admin/pycheck
  doc
  doc/Makefile
  doc/addressbook-slides-server.tac
  doc/addressbook-slides.xml
  doc/chat-bind.dia
  doc/chat-search-pipeline.dia
  doc/chat-search.dia
  doc/chat-unbind.dia
  doc/example-objectclass.xml
  doc/examples
  doc/examples/addressbook
  doc/examples/addressbook/01_repl
  doc/examples/addressbook/01_repl/session-01
  doc/examples/addressbook/01_repl/session-02
  doc/examples/addressbook/01_repl/session-03
  doc/examples/addressbook/01_repl/session-04
  doc/examples/addressbook/01_repl/session-05
  doc/examples/addressbook/01_repl/session-06
  doc/examples/addressbook/01_repl/session-07
  doc/examples/addressbook/01_repl/session-08
  doc/examples/addressbook/01_repl/session-09
  doc/examples/addressbook/01_repl/summary
  doc/examples/addressbook/02_script
  doc/examples/addressbook/02_script/addressbook.py
  doc/examples/addressbook/02_script/summary
  doc/examples/addressbook/03_webapp
  doc/examples/addressbook/03_webapp/addressbook.py
  doc/examples/addressbook/03_webapp/addressbook.tac
  doc/examples/addressbook/03_webapp/run
  doc/examples/addressbook/03_webapp/summary
  doc/examples/addressbook/04_dynamic
  doc/examples/addressbook/04_dynamic/addressbook.py
  doc/examples/addressbook/04_dynamic/addressbook.tac
  doc/examples/addressbook/04_dynamic/run
  doc/examples/addressbook/04_dynamic/summary
  doc/examples/addressbook/05_form
  doc/examples/addressbook/05_form/addressbook.py
  doc/examples/addressbook/05_form/addressbook.tac
  doc/examples/addressbook/05_form/run
  doc/examples/addressbook/05_form/searchform.xhtml
  doc/examples/addressbook/05_form/summary
  doc/examples/addressbook/06_pretty
  doc/examples/addressbook/06_pretty/addressbook.py
  doc/examples/addressbook/06_pretty/addressbook.tac
  doc/examples/addressbook/06_pretty/run
  doc/examples/addressbook/06_pretty/searchform.xhtml
  doc/examples/addressbook/06_pretty/summary
  doc/examples/addressbook/07_easy
  doc/examples/addressbook/07_easy/addressbook.py
  doc/examples/addressbook/07_easy/addressbook.tac
  doc/examples/addressbook/07_easy/run
  doc/examples/addressbook/07_easy/searchform.xhtml
  doc/examples/addressbook/07_easy/summary
  doc/examples/addressbook/08_integrated
  doc/examples/addressbook/08_integrated/summary
  doc/examples/addressbook/09_maintainable
  doc/examples/addressbook/09_maintainable/summary
  doc/examples/addressbook/10_debianized
  doc/examples/addressbook/10_debianized/summary
  doc/examples/addressbook/README.txt
  doc/examples/addressbook/server
  doc/examples/addressbook/server/addressbook.schema
  doc/examples/addressbook/server/doe.ldif
  doc/examples/addressbook/server/exam.ldif
  doc/examples/addressbook/server/initial.ldif
  doc/examples/addressbook/server/run
  doc/examples/addressbook/server/slapd.conf
  doc/examples/addressbook/server/smith.ldif
  doc/examples/global.cfg
  doc/examples/ldap_parallelsearch.py
  doc/examples/ldif2ldif
  doc/interfaces.dia
  doc/ldap-intro.xml
  doc/ldap-is-a-tree.dia
  doc/ldapentry-vs-oo.xml
  doc/ldapfilter-as-tree.dia
  doc/overlay.js.patch
  doc/overview.dia
  doc/search-inputs.xml
  doc/slide-style.css
  doc/slides-driver.xsl
  doc/tags.j2h
  ldaptor.schema
  test-ldapserver.tac
```
as for the rest, my opinions are as following:

- `doc`: nope, this is big and confusing and not really helpful.
- `admin`: is anything of that used and/or useful?  Shipping useless stuff is confusing to the users.  We should at least kill off pycheck, doctest-all.py belongs into tox so should be killed too. Remains `l10n-update` which is potentially used by parts I haven’t ever touched so I have absolutely no idea whether it’s used/useful.
- `test-ldapserver.tac`: seems at least to start. Ship for now, but we should create an examples directory eventually.
- `ldaptor.schema`: not sure what it exactly does but ship it for now.