setup: drop support for Python 2.6

It would be a a stepping stone for the migration to Python 3 to only support
Python 2.7. Even though we don't make any big changes now, it might allow us to
remove some workarounds or use some new forward-compatible features.

Mercurial dropped support for Python 2.6 2 years ago.

setup: move test dependencies to dev_requirements.txt to make them optional

Remove the need for having test tools on production systems. Installing test
dependencies is made an extra explicit step.

pip is the future, but doesn't have the same tests_require features as
setuptools kind of has.

I don't like this way of handling it without setup.py support and with explicit
naming of the ugly dev_requirements.txt ... but that seems to be the way to do
it.

tox.ini: declare 1.8 as minimum tox version

Older tox versions will fail with
tox.ConfigError: ConfigError: substitution key '26,27' not found

Older tox versions will apparently not understand minversion anyway, but
at least it states the intent ...

tests: Move pytest config (norecursedirs) from tox.ini to setup.cfg

Avoid (conceptual) dependency on the tox meta test runner configuration.

tox.ini: Add .eggs to norecursedirs

This prevents py.test from trying to run tests in the .eggs directory, which
setuptools can create.

tests: add pytest section and put tox and tests/scripts to norecursedirs

kallithea/tests/scripts are non-integrated messy tests
that cant be part of a testsuite as is

tox.ini: Set PYTHONHASHSEED = 0

because there is some non-deterministic stuff that can fail without this