[oe] [STABLE] Updating Python code to work nicer with Python 2.6

[Marcin Juszkiewicz]

Some time ago I fixed BitBake trunk to not generate DeprecationWarnings
when used with Python 2.6. Some time later Richard Purdie backported it
into bitbake-1.8 branch. Now I tested it with stable/2009 and want to
get it inside.

Patch.bbclass change removes DeprecationWarnings too but has to keep
compatibility with Python 2.4 so I had to use try/except block because
hashlib is 2.5+