Drop Python 3.5 support

[snowman2]

Seems Python 3.5 support is being dropped in the community.

• numpy
• pandas
• rasterio
• xarray
• datacube
• Django 3

Also 3.5 has not been supported by conda/conda-forge for a while.

I am wondering how many Python 3.5 users are out there and how long pyproj should continue support. Anyone have insight they would like to share?

[snowman2]

EOL for Python 3.5 is Sept. 2020:
https://devguide.python.org/#status-of-python-branches