It looks like this history survived the cvs to svn migration, but was lost in migration to hg.
It's not there anymore, but it isn't lost, either - you can still get it from svn if you want to (and should still be able to do so twenty years from now if you care - just as you can still get the original CVS repository if you want to).
In any case, this part of the history has not been converted, and there is no way to fix it now - less so in Mercurial than there might have been in CVS. Changing the history would require to come up with completely new revision "numbers", which would be unrelated to the revision numbers in all the clones out there. So anybody pushing changes would bring back all these "incorrect" revisions. That's why people say that changing history is futile in DVCSs one the genie is out of the bottle.
So just accept that apparently, you wrote datetime.py all on your own :-)
If you wonder what precisely has gone wrong: you shouldn't have created a branch in /sandbox, but in python/branches. /sandbox shouldn't have been used for anything whose history is of any interest. The hg conversion didn't consider anything outside /python, so it couldn't track the copying of the file.
It might be that the svn-to-hg conversion algorithm could do better with files that are copied from within the same repository, but from outside the converted subtree. This is tricky to implement, though, since you can't give reasonable file path names to such files. OTOH, our approach to svn (i.e. multiple projects in one repository) is so uncommon that the Mercurial people are likely to ignore problems arising out of it.