[FEATURE REQUEST] Add full support for Python 3.9

[btjanaka]

Description

Currently, numba does not work with Python 3.9 (numba/numba#6345), so we cannot support it yet.

This is due to numba/llvmlite#669. Since Python 3.9 wheels are not available for llvmlite, CI/CD is trying to build the wheels, but is unable to due to lack of llvm. This should be resolved in a few weeks. For now, Python 3.9 will only work if users can build the llvmlite wheel on their own -- for this, they will need to have the llvm libraries installed.