Cope with unicode __all__ in Python 2 packages

When porting a package from Python 2 to 3, one natural path involves
adding `from __future__ import unicode_literals` everywhere to prepare
for the `bytes`/`unicode` change.  This can cause `__all__` to contain
`unicode` elements, which mostly works but breaks star imports as
follows (depending on the exact Python version - see
https://bugs.python.org/issue21720):

    TypeError: Item in ``from list'' not a string

    TypeError: Item in ``from list'' must be str, not unicode

Star imports can usually be avoided, but it's hard to avoid this
behaviour of zope.configuration if you're using ZCML, so it seems worth
adjusting `ConfigurationContext.resolve` slightly to avoid the problem.
The `sys.modules` logic is borrowed from 2.7's
`importlib.import_module`.  (Using `importlib` directly here is tricky
because of the care we take with tracebacks, but for the time being we
can still get by with `__import__`.)

CHANGES.rst

@@ -4,8 +4,8 @@ Changes
 4.1.1 (unreleased)
 ------------------
 
-- Nothing changed yet.
-
+- Fix resolving names from a Python 2 package whose `__init__.py` has
+  unicode elements in `__all__`.
 
 4.1.0 (2017-04-26)
 ------------------

src/zope/configuration/config.py

@@ -42,8 +42,6 @@
 metans = 'http://namespaces.zope.org/meta'
 testns = 'http://namespaces.zope.org/test'
 
-_import_chickens = {}, {}, ("*",) # dead chickens needed by __import__
-
 
 class ConfigurationContext(object):
     """Mix-in that implements IConfigurationContext
@@ -148,7 +146,8 @@ def resolve(self, dottedname):
             oname = ''
 
         try:
-            mod = __import__(mname, *_import_chickens)
+            __import__(mname)
+            mod = sys.modules[mname]
         except ImportError as v:
             if sys.exc_info()[2].tb_next is not None:
                 # ImportError was caused deeper
@@ -167,7 +166,9 @@ def resolve(self, dottedname):
         except AttributeError:
             # No such name, maybe it's a module that we still need to import
             try:
-                return __import__(mname+'.'+oname, *_import_chickens)
+                moname = mname + '.' + oname
+                __import__(moname)
+                return sys.modules[moname]
             except ImportError:
                 if sys.exc_info()[2].tb_next is not None:
                     # ImportError was caused deeper

src/zope/configuration/tests/test_config.py

@@ -114,6 +114,23 @@ def test_resolve_bad_sub_import(self):
            if name in sys.modules:
                del sys.modules[name]
 
+    def test_resolve_unicode_all(self):
+        # If a package's __init__.py is in the process of being ported from
+        # Python 2 to Python 3 using unicode_literals, then it can end up
+        # with unicode items in __all__, which breaks star imports from that
+        # package; but we tolerate this.
+        import sys
+        c = self._makeOne()
+        self.assertEqual(
+            c.resolve('zope.configuration.tests.unicode_all').foo, 'sentinel')
+        self.assertEqual(
+            c.resolve('zope.configuration.tests.unicode_all.foo'), 'sentinel')
+        # Cleanup:
+        for name in ('zope.configuration.tests.unicode_all',
+                     'zope.configuration.tests.unicode_all.__future__'):
+            if name in sys.modules:
+                del sys.modules[name]
+
     def test_path_w_absolute_filename(self):
         import os
         c = self._makeOne()

src/zope/configuration/tests/unicode_all/__init__.py

@@ -0,0 +1,5 @@
+from __future__ import unicode_literals
+
+__all__ = ['foo']
+
+foo = 'sentinel'