Final improvements before uploading v5.0 to PyPi

Add proper Python classifiers. Test with Python 3.9 in CI.

Bump to version 4.4

Merge branch 'master' into 4.x-maintenance

Bump to version 4.2

Bump to 4.1

Fix classifiers

Merge branch 'master' into 3.x-maintenance

Bump to version 3.0

Bump to version 2.1

Fix README, "include" is "searchpath"