Zserio v2.1.0-pre1

This is the first beta version of the zserio v2.1.0 which contains improved python emitter generating type annotations.

The pre-release contains several fixes as well.

Fixes

#237 - Remove postfixes from 32-bit literals in C++ generated code
#240 - Emitted python code for fromFields method is incorrect in case of an optional array

Improvements

#197 - Python: Type annotations for generated code
#235 - Python: Don't generate empty parentheses in a class definition
#236 - Python: Type annotations for runtime library
#239 - Update google test library to the newest version 1.10.0