Entrypoints for pandas >=0.25

[jsignell]

Doesn't yet return proper help or work with df.plot.scatter() style calls

EDIT: it does work with df.plot.scatter() style calls, they just have the same requirements as the df.plot versions (aka plot.scatter requires x and y to be set explicitly whereas hvplot does not).

Closes #277

[jsignell]

I consider this done. There are some outstanding issues to be resolved on the pandas side. But this chunk of work should probably get in sooner rather than later.

[jsignell]

Note that in this PR, by, groupby, and hover_cols will only work if the columns that they reference are in x or y. See pandas-dev/pandas#28647 for the full description.

[jsignell]

To Do:

• make sure that there are no references to patch in the docs

[jsignell]

I am fairly confident that the remaining error is unrelated.

[jsignell]

I think this is ready to merge