Fix building on FreeBSD

Cirrus CI on FreeBSD failed because "No packages available to install
matching 'py37-pip' have been found in the repositories"i [1].
Package 'py37-pip' has been replaced with 'py38-pip', see [2].

1. https://cirrus-ci.com/task/6375178809638912
2. https://www.freshports.org/devel/py-pip/

readme: add references and similar projects

Closes #47