make ZServer a dependency for py2 only

[pbauer]

Fixes #6

[ale-rt]

I wonder if we should just drop the requirement for all Python versions.
It seems ZServer is not used in the package at all:

[ale@emily Products.SiteErrorLog]$ grep -R ZServer
setup.py:        'ZServer',

[tseaver]

The (really ugly) testSiteErrorLog module imports Zope2 and calls Zope2.startup() at import time, which fails if ZServer is not installed.

That has to be bogus: no reasonable test needs to have the server running.

[icemac]

@hannosch Is this package intended to run with Zope 4 thus should it be ported to Python 3?

[icemac]

@ale-rt Running the test on Python 3.6 breaks with ModuleNotFoundError: No module named 'ZServer' as Zope2.startup() is actually ZServer.Zope2.startup().

[ale-rt]

Oh, I see @icemac.
So actually we can move this requirement to the [test] feature, even better fix the test to not use Zope2.startup().
Anyway it seems reasonable to me to merge this one

[tseaver]

I don't think we should merge as-is. Instead, we should fix the stupid test which starts a server at import time, and remove the ZServer dependency altogether.

[icemac]

@tseaver Do you think this is mergeable after my fixes?