[Python 3] def ensure_binary / some ___prepare binary return fix

Summary:
def ensure_binary added

"== None" -> new PEP8 style "is None"

def lsub ensure ml is str for variable manipulation
def lm prepare return bytes
manipulations and clauses between prepare and docommand ensure to be bytes
as well, as var1, var2 = docommand(), var2 ensure_str

Reviewers: PyKolab Developers

Subscribers: mollekopf, sicherha

Differential Revision: https://git.kolab.org/D3863