Python and Transcrypt Version Update

[JennaSys]

At the time the React to Python book was written, Transcrypt only worked with Python version 3.7. However a new version of Transcrypt was recently released that works with Python 3.9. This is now the default version on PyPI, and if no version is specified when you use PIP to install it, you will get the newer 3.9 version. If you are are using Python 3.7 as the book describes, this will likely result in compilation errors. So you now have two options:

1. Continue to use Python 3.7 in which case you will have to specify the version of Transcrypt to use when you install it:
   pip install transcrypt==3.7.16
2. Use Python 3.9 instead of Python 3.7 when running the examples and just install the latest version of Transcrypt:
   pip install transcrypt

While not all of the examples from the book have been fully tested with Python 3.9, they should be compatible with it. Other than installing the correct version of Transcrypt based on the Python version you are using, no other changes should be needed.