Update Makefile

[ezio-melotti]

• make clean should also remove the venv (see Update venv and clean make targets #856)
• make clean-venv should be added to only remove the venv (see Update venv and clean make targets #856)
• make check should check for the default role (see Use sphinx-lint in make check. #866)
• make htmlview should be added (see bpo-36329: Replace 'make serve' with 'make htmlview' #826)
• make venv shouldn't print when called by other targets (see Only print make venv message when needed. #867)

[ezio-melotti]

This is all done -- closing.