Issues with package concurrencytest in chapter 19

[Matej-Chmel]

Package concurrencytest that is imported in Chapter19/example7.py can't be used on Windows, because the default interpreter is missing implementation of the os.fork function. When running the example under Windows the following error is generated:

AttributeError: module 'os' has no attribute 'fork'

Also the package is missing from software list in README.