[fix] Support local version label for zc.buildout

When there is no pinned version for zc.buildout itself, buildout
adds a ">=<current-version>" requirement to prevent accidental
downgrading. If the current version has a local version label,
this produced an invalid version specifier. To fix this, only
the public part of the current version is used.

src/zc/buildout/buildout.py

@@ -480,7 +480,8 @@ class Buildout(DictMixin):
                      ('zc.buildout',
                       '>='+pkg_resources.working_set.find(
                           pkg_resources.Requirement.parse('zc.buildout')
-                          ).version),
+                          # Skip local part because ">=x.y.z+abc" is invalid
+                          ).parsed_version.public),
                      # Use 2, even though not final
                      ('zc.recipe.egg', '>=2.0.6'),
                      )