Remove references to Python 2 installations

[blakerosenthal]

Checks for sys.version_info < (3,) are no longer necessary:

https://github.com/ActivitySim/activitysim/blob/a3dca1f8280374cc3fa22aa27d0503a20f3bdf58/activitysim/core/config.py#L314 https://github.com/ActivitySim/activitysim/blob/a3dca1f8280374cc3fa22aa27d0503a20f3bdf58/activitysim/abm/tables/skims.py#L40 https://github.com/ActivitySim/activitysim/blob/a3dca1f8280374cc3fa22aa27d0503a20f3bdf58/activitysim/core/steps/output.py#L43

We can also consider updating our string interpolation convention to use the more readable 'fstring' format, only in Python 3: https://www.python.org/dev/peps/pep-0498/ https://realpython.com/python-f-strings/