Python: Fix missing module resolution

This was due to bad manual magic: restricting the attribute name makes
sense when we're talking about submodules of a package, but it doesn't
when we're talking about reexported modules.

Also (hopefully) fixes the tests so that the Python 3-specific bits are
ignored under Python 2.

python/ql/lib/semmle/python/dataflow/new/internal/ImportResolution.qll

@@ -235,9 +235,12 @@ module ImportResolution {
       result = ar
     |
       isPreferredModuleForName(m.getFile(), p.getPackageName() + "." + attr_name + ["", ".__init__"])
-      or
-      // This is also true for attributes that come from reexports.
-      module_reexport(p, attr_name, m)
+    )
+    or
+    // This is also true for attributes that come from reexports.
+    exists(Module reexporter, string attr_name |
+      result.(DataFlow::AttrRead).accesses(getModuleReference(reexporter), attr_name) and
+      module_reexport(reexporter, attr_name, m)
     )
     or
     // Submodules that are implicitly defined with relative imports of the form `from .foo import ...`.