Update NVDA build environment to Python 3

[LeonarddeR]

To create a distribution of NVDA, several third party packages are used and some scripts are involved. In particular:

• Py2exe to byte compile Python code and collect dependencies: Update setup script for Python 3.7 #9605 (Update setup script to be compatible with Py2exe for Python 3.7 #8375)
• txt2tags to build the t2t documentation: Python 3: find a suitable and compatible replacement for txt2tags #8734
• Epydoc: used to build developer documentation for our Python code
• Doxygen: Used to create the developer documentation for NVDAHelper
• Robot Framework: Used for our system tests
• SCons: to offer the environment that executes and bundles just everything we need

Most runtime dependency updates are handled in #9630.
I started to work on the conversion of our SCons build environment to Python 3. For now, I disabled the ability to generate developer documentation. For txt2tags, it turns out that the converted package by @nishimotz does work. At this stage, I don't think that we should block the Python 3 transition on a possible switch to markdown or another format for our documentation.

For now, I'd like to propose creating something that is able to create an NVDA distribution, not more than that. This means that, apart from the t2t developer guide, I also vote for commenting out the ability to create developer documentation. This means that we can focus on something that actually builds and works, without being distracted by the need to deliver developer documentation. Restoration of developer documentation creation can be done in subsequent pull requests.

This means that currently, I can identify only one major show stopper, namely txt2tags. Therefore, I think we should consider looking into the work by @nishimotz.

I consider this to be an umbrella issue for the build framework update, particularly aimed at the work for SCons.

[LeonarddeR]

Apart from some small py2exe issues and #9840, this work has been completed. Please open separate issues for any other issues you may encounter in NVDA's build environment.