DOC: whats new v0.20

[greglucas]

This adds a what's new entry for v0.20.

Additionally, I added a commit that removes the Python 2 warning, and changes that to a note about how to get a speedup with PYPROJ_GLOBAL_CONTEXT, which I think is important to make people aware of.

Things to do before the v0.20 release still:

• Pcolormesh update for interpolation with wrapped coordinates #1646
• Use packaging.version instead of distutils #1843
• Bump minimum Proj to 8.0.0 #1854

Things to consider:

• Where should we put the new documentation? Keep the old location and just live without the version selector for now, or try to move hosting to RTD (Move the documentation to ReadTheDocs? #1765)
• Removing the PROJ dependency entirely. (Port Geodesic routines to use pyproj #1832)

[QuLogic]

Please also add the dependency changes.