Some more code cleanups

[syntron]

additional cleanups which should be added before #257

• Remove unreachable code
• cleanup old code for Python 2.x
• fix OMCSessionBase - the variables self.readonly and self.omc_cache are needed here
• [ModelicaSystem] fix True in docstring
• [OMCSessionBase] fix usage of excaptions - replace ex.message() by ex.msg()

[syntron]

see also discussion in #250 - small changes to make OMCSessionBase and OMCSessionZMQ more independent of each other