Fixed __builtin__ confusion from commit 1883077

Commit 1883077 did ```import __builtin__``` on Python 2 and ```import builtins as __builtin__``` on Python 3, but used that module as ```builtins``` later in the code. See https://github.com/espressomd/espresso/commit/188307706f2abe3212619a11cd0644fadbb3d58e#diff-19