Add support for python 3.9

[sarahmish]

The changes involved are:

• Change the setup.py python specifications
• Update copula to the latest version
• Add Python3.9 to tox.ini
• Add Python3.9 to .github/workflows/tests.yml
• Add 3.9 to the list of supported Python versions in the README.md
• Any other changes required in the code that may show up during testing

One issue I faced with copula==0.6 is that percent_point only supports 1d arrays which consequently makes some unittests in GaussianCopulaProcessMetaModel fail.