Building Tensorflow 1.5
Update: I installed python 3.6 alongside the default 3.8 and then installed tensorflow 1.5 and it looks like it works now (albeit with a few 'future warnings'.)
I have an old Macbook Pro 3,1 running ubuntu 20.04 and python 3.8. The mac CPU doesn't have support for avx (Advanced Vector Extensions) which is needed for tensorflow 2.2 so whilst tensorflow installs, it fails to run with the error:

illegal instruction (core dumped)

I've surfed around and it seems that I need to use tensorflow 1.5 however there is no wheel for this for my configuration and I have the impression that I need to build one for myself.
So here's my question... how do I even start to do that? Does anyone have a URL to Building-Stuff-For-Dummies or something similar please? (Any other suggestions also welcome)
Thanks in advance for your help