Update doc requirements and validate them on all supported Python versions

[vkarak]

Currently we Python 3.8 which holds up back in upgrading Sphinx and some other doc packages. Since the docs of all reframe versions + the latest dev are online, we don't need to keep us back in a specific Python version for building the docs. We should also make sure to update the README about the minimum version required to build the docs.

[vkarak]

This is not valid as building the docs is part of the installation process that will generate the man pages (./bootstrap.sh +docs), so it has to work on all supported Python versions. I will revert the issue's title to its original one.