Drop Python 3.6 and improve ipyleaflet tile layer

Bye-bye, Python 3.6! 👋🏻

This release introduces an override to ipyleaflet.TileLayer that constrains the region of requested tiles, per jupyter-widgets/ipyleaflet#888. This significantly reduces the load on the tile server when viewing tiles in an ipyleaflet.Map

This change required dropping Python 3.6 and was good timing since Python 3.6's Security Support ended Dec 23, 2021 (4 days ago) (ref https://endoflife.date/python). Since conda-forge hasn't been building on Python 3.6 for some time, I'm hoping there is minimal impact here