Python 3 - Don't use `commands` module anymore.

The `commands` module was deprecated in 2.7 and removed in 3.x.
As a workaround, we introduce a new module `seven` in
lldbsuite.support, and write helper functions in there that delegate
to the commands module if it is available, and re-implement their
functionality for cases where it is not available.

llvm-svn: 251959