Switched from deprecated "setup.py install" to "pip install ."

[radarhere]

Replaces setup.py install with pip install ., as per https://blog.ganssle.io/articles/2021/10/setup-py-deprecated.html

As worked out by pypa/setuptools#2929, this also fixes the problem found in #5886. So this PR reverts #5886 and #5888.

[hugovk]

Good stuff!

There's some setup.py develop as well, can they be pip install -e?

Also some setup.py build, setup.py build_ext and setup.py clean but they can wait.

[radarhere]

Ok, done.

[hugovk]

Thanks!