Ensure the typing package is installed only if needed (python < 3.5)

[WuShell]

This prevents problems when installing cortex4py in recent python versions, where typing is installed as an external package, while there is a typing module included in the stdlib.

The external package then "shadows" the stdlib module, breaking other packages that use/rely on typing (when using cortex4py in a project with more dependencies)

I've used the suggested "incantation" to set up typing as a required dependency from the project site, here:

https://pypi.org/project/typing/

"For package maintainers, it is preferred to use typing;python_version<"3.5" if your package requires it to support earlier Python versions. This will avoid shadowing the stdlib typing module when your package is installed via pip install -t . on Python 3.5 or later."

Tested locally with python 3.7, 3.8 and 3.9

[sinnwise]

Hey, just wanted to bump this one if possible