Revert "Remove the imports from __init__ as they are not needed and will make setup.py fail if six is not installed."

This reverts commit 33eafa8b25335420f64abdc3b6d173d4b7c7124d.
Prevent the attempt to import required but not yet installed modules by specifying the version number in setup.py instead of importing it from javalang/__init__.py.

Signed-off-by: David Black <dblack@atlassian.com>
Merge branch 'python_3_support' into minor_code_fixup