Properly declare dependency on zope.schema >= 4.2.0

[cjwatson]

This was introduced in zope.security 4.2.1.

[cjwatson]

I happened to run into this when upgrading Launchpad's dependencies from fairly old versions, getting a traceback along the way that ended in:

Traceback (most recent call last):
...
  File "/home/cjwatson/src/canonical/launchpad/git/launchpad/env/local/lib/python2.7/site-packages/zope/security/interfaces.py", line 56, in <module>
    from zope.schema import Text, TextLine, NativeStringLine
ImportError: cannot import name NativeStringLine

It would be a bit friendlier to declare this.

[jamadden]

LGTM.

It looks like I originally introduced the import of NativeStringLine without bumping the dependency version a couple years back. zope.schema 4.2 was around four years old at that point so I guess it didn't even occur to me that adding a version pin would be helpful.

Existential thoughts that can be ignored: zope.schema is at version 4.9.3 now, and that's what CI is testing, so it occurs to me to wonder if 4.2 is a sufficient enough pin. Most likely it will rarely be tested in that exact configuration. There's a tension between end-user ease of deployment and ease of development here. It's a hard problem. None of this is new of course. Thanks for fixing my error! I'm sorry you had to deal with it.

[mgedmin]

Thank you for caring!

[cjwatson]

Yeah, I don't have a better solution than either (a) bump versions when we notice problems or (b) a huge tox matrix to test the minimum version pins. Probably not worth the effort for (b) ...