Ignore Python version for eggs in part signatures

The version of Python should not affect the behaviour of a recipe.
Anyway, it was already ignored for DEVELOP_DIST eggs.

This makes the slapos mechanism to share parts more efficient. And with
the upcoming changes in buildout & slapos.recipe.cmmi, there would be
no way for the slapos.reboostrap extension to prevent everything from
being rebuilt when reboostrapping to a different version of Python,
if bootstrap parts are shared.

The monkey-patch by slapos.reboostrap now becomes useless.

src/zc/buildout/buildout.py

@@ -1935,7 +1935,7 @@ def _dists_sig(dists):
         if dist.precedence == pkg_resources.DEVELOP_DIST:
             result.append(dist.project_name + '-' + _dir_hash(location))
         else:
-            result.append(os.path.basename(location))
+            result.append(dist.project_name + '-' + dist.version)
     return result
 
 def _update_section(s1, s2):