Fix compatibility with Python 2.6
Fix compatibility with Python 2.6

Python 2.6 doesn't have dict comprehensions;
use dict(<generator expression>) instead.

Fixes:

   Traceback (most recent call last):
     File "test/runtest.py", line 136, in <module>
       tst, all = suite(dirs, clean=clean, pre=pre, verbose=verbose)
     File "test/runtest.py", line 70, in suite
       mod = __import__(test, globals(), locals(), [mod])
     File ".../test/muppy/test_muppy.py", line 7, in <module>
       import pympler.muppy
     File ".../pympler/muppy.py", line 3, in <module>
       from pympler import summary
     File ".../pympler/summary.py", line 147
       right_by_key = {r[0]:r for r in right}
                                ^
   SyntaxError: invalid syntax

introduced in 5da73b5b19581a7ab9ca8810fd16ba5e6835c536.

pympler/pympler,pympler/pympler