hg st/clone/pull crashes on old machine with 5.4

[Marcin Kasperski]

> I  expect that the patch will not be committed for the reasons stated in
> the patch itself. …

Mayhaps modern Mercurial should simply refuse to build and install on
2.7.3 ?   

Clear message about unsupported python version (with suggestion to
eitherupgrade python or use such-and-such old mercurial) would be
friendlier than mysterious crash…