Thrift-1806: Python generation always truncates __init__.py files

Thrift-1806: Python generation always truncates __init__.py files
Client: Python
Patch: Nathaniel Cook

When generating python code the generator correctly creates _init.py file for each directory. If the __init_.py file has contents it is always overwritten. This usually isn't a problem unless using the -out flag, then there is a potential to erase non generated code.