* rename requirements.txt to requirements-py2.txt, to make it clear they are Python 2-only
* make requirements-py3.txt consistent with requirements-py2.txt

Using https://github.com/lopuhin/pydispatcher, pypy branch.
This is executed as a separate step to avoid changing
default requirements.txt and setup.py. If just added to "deps"
in tox, this install command will be executed as one command
and PyPyDispatcher will not override PyDispatcher.

See http://www.sphinx-doc.org/en/stable/changes.html#release-1-4-released-mar-28-2016

sphinx_rtd_theme has become optional, needs to be added to reqs

https://github.com/sphinx-doc/sphinx/pull/2320 changes node entries tuples
to 5 values instead of 4

`sh` syntax highlighting added very locally in selectors.rst
because of this warning/error with Sphinx 1.4:

```
Warning, treated as error:
/home/paul/src/scrapy/docs/topics/selectors.rst:743:
WARNING: Could not lex literal_block as "python". Highlighting skipped.
```

This allows to remove `get_testlog` helper, `flushLoggedErrors` from
twisted.trial.unittest.TestCase and Twisted log observers created for
each test on conftest.py.

Also, Twisted version is bumped for Python 3.x tests, just in case.

service_identity is not added to Scrapy requirements because Scrapy
supports older Twisted / PyOpenSSL versions which don't use it.

mock is in the stdlib since Python 3.3.