pure module loading issues with stable tip

[Steve Borho]

Hello,

obsolete.py is importing the base85 module with a "from" statement:
from mercurial import util, base85

If one is using the pure Python extensions by adding mercurial/pure to
sys.path, this import statement breaks.

It can be trivially fixed but I wasn't sure if using the pure modules
this way was intended to be a supported feature.

-- 
Steve Borho