Drop support for Python 3.5

[ivanst0]

Describe the solution you'd like
Now that Python 3.5 is officially retired, we should stop supporting this version of Python in onnxruntime codebase. This will
- remove some version checks from the code
- enable using the language features introduced four years ago (in Python 3.6)
- reduce the CI build matrix (or at least keep it same size, since Python 3.9 will need to be added very soon).

Additional context
It should be ok to do this in a minor release (onnxruntime 1.6). For reference, PyTorch dropped support for Python 3.5 in version 1.6, released on 7/28/2020.

[snnn]

But Ubuntu 16.04 still need it. I think we may wait a few month. Ubuntu 16.04 will be EOL on April 30th 2021.