Run flake8 in Github CI on target Py version

CI/README: master->main

No matplotlib in examples

Fix visualization of ellipse example

See merge request !22

Fix a logging message in conformal-mapping.py

See merge request !21

Conformal mapping example

* Mac CI: is gfortran there?

* Mac CI: is gfortran there?

* Try to find gfortran via gcc

* Try to find gfortran via gcc

* Blindly try gfortran-9 as a Fortran compiler

* More shots in the dark for gfortran on Github

* Use F77/F90 env vars to point setuptools on Mac Github CI towards a Fortran comiler

June monster maint

macOS CI: Switch to Python 3, use Github Actions