Building with Python 3.8 (or higher?)

In Python 3.8 `python3-config --ldflags` does not contain the flag to link with libpython anymore.
Use `python3-config --ldflags --embed` instead.

For example:
```
gfortran -c forpy_mod.F90
gfortran intro_to_forpy.F90 forpy_mod.o `python3-config --ldflags --embed`
```