Drop Python 3.5 support and start supporting Python 3.9

[csala]

On September 13th 2020 Python 3.5 reached its end of life, so we can stop explicitly supporting it.
On the other side, on October 5th, Python 3.9 was released, so we should add it to our supported list.

[csala]

Python3.5 has already been dropped, but Python3.9 still needs to be added.

The changes involved are:

• Change the setup.py python specifications
• Review the dependency ranges to ensure compatibility
• 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

[gboehl]

I can confirm that the package builds and runs with Python 3.10 (also in development mode).

Why did you add the maximum versions to each package in setup.py (i.e. "<2")? The standard should be to only specify a minimum version number and specify the max version only where strictly necessary. The maximum number are messing with the packages installed by pip, as documented in #259. For my 3.10 build I removed all the maximum numbers...

[npatki]

Seems like this issue is now obsolete. As of the current version (Copulas 0.7.0) the Python requirements are >=3.6 and <3.10.

Let's use sdv-dev/SDV#1000 to track the general issue of removing the maximum caps to Python 3.