… available

As requested by maintainer

Since the new Ubuntu 20.04  LTS version is shipping
python3.8 as the default python, update the Makefile
to use that as default and more importantly fix the
python-config invocation to work with all python
versions.

The fix might be more widely required, but in particular
python3.8 would segfault when PyUnicode_FromString() was
called before the interpreter was initialized.

Which is expected tbh, but a change in behaviour from
earlier versions.