wip-feat: pandas as soft dependency

[mattijn]

This PR is an attempt to make pandas a soft dependency. I hope it can be used as inspiration, as I was not able to make the types happy. I've no real idea how it should be done, but I've been trying a few things, some with success and others without.

I also made an attempt to prioritize the DataFrameLike approach over the pandas routine, but decided to not do this as otherwise usage of a pandas DataFrame within Altair will require pyarrow to infer/serialize. My current feeling is that usage of pandas to infer and serialize the data is still preferred as it is not yet depending on pyarrow.

[binste]

Great to get the ball rolling on this, thank you @mattijn! I did not yet have time to review but just wanted to say that I'm happy to have a look at the types once I get to it. As long as the package works, I'm optimistic that we can make mypy happy.

[mattijn]

Thanks @binste! No rush! Maybe something for version 5.4