Force python2 usage and provide Ubuntu install scripts for Focal

[neilsummers]

I ran into the same issue as #13 and have been trying to find a good solution since upgrading to Ubuntu Focal 20.04. Previously had it working on Ubuntu 18.04 (Bionic) with no problems.

Python2 and Python3 can both coexist on the same system without conflict as all libraries are installed in separate site-packages directories and only the Python? directories are used in the Python path when invoking the correct interpreter version. It is Ubuntu's package management system that will not allow both to coexist, by using the python-is-python? package to force one version or the other. You can get this to work quite easily by adding bionic sources and using python-is-python2 and then continuing with install as before, but then everything on your system is limited to python2, which is not a good solution.

Therefore I managed to install fwbackups by installing the dependencies and hacking the dependencies file to remove the dependencies which forced the python2 version. Then with just some simple modifications to the fwbackups scripts to point explicitly to python2, and making sure python2 is specified when running configure, and everything now worked as usual. The method was quite tricky so I made a script to automate it and made this PR for visibility in case anyone else wants to follow the same procedure.