linking python libraries fails in focal

You don't need a separate machine. https://wiki.ubuntu.com/SimpleSbuild should let you set up a build environment for focal on your bionic system, and then you can debug the problem interactively. Try that first; if you're still stuck after that then we can possibly have a look.

I'll be damned..

pkg-config is not correctly configured for python3-dev in ubuntu 20..!

In LTS 18:

pkg-config python3 --libs

gives

-lpython3.6m

In LTS 20:

it gives nothing..!

I wonder where should I file a bug report about this one..

ah.. it's been mentioned here:

https://askubuntu.com/questions/1256991/python-3-shared-libraries-and-pkg-config-on-ubuntu-20-04-lts

OK - that looks like it should give you enough to go on, so marking this as Answered again, unless there's more for us to do.

I do this cmake bs in my CMakeLists.txt:

exec_program(python3-config ARGS --includes OUTPUT_VARIABLE PYTHON_INCLUDE_DIRS_TMP) # this bullshit must be parsed
exec_program(python3-config ARGS --libs OUTPUT_VARIABLE PYTHON_LIBRARIES_TMP)
string(REPLACE "-l" " " PYTHON_LIBRARIES_TMP ${PYTHON_LIBRARIES_TMP})
string(REPLACE " " ";" PYTHON_LIBRARIES ${PYTHON_LIBRARIES_TMP})
string(REPLACE "-I" " " PYTHON_INCLUDE_DIRS_TMP ${PYTHON_INCLUDE_DIRS_TMP})
string(REPLACE " " ";" PYTHON_INCLUDE_DIRS ${PYTHON_INCLUDE_DIRS_TMP})

seems to fix the issue at least on my ubuntu 20 laptop

python3-config --libs

and

python3-config --ldflags

Give completely different answers in 18.04 and 20.04 ...!

In 18.04 the correct answer is given with "--libs" while in 20.04 it should be "--ldflags"

Any help appreciated..

Sorry, at this point this is outside the realm of things we can help you with as Launchpad support people; we just make build environments available, and don't maintain Ubuntu's Python packages. I suggest asking on askubuntu.com, or filing a bug using "ubuntu-bug python3.8" if you believe it to be a bug in the 20.04 Python packaging.