Drop None from Container.get_children().

[anntzer]

Before the patch,

ec = plt.errorbar([1, 2], [3, 4], [5, 6], capsize=2, fmt="none")
print(ec.get_children())

would print None (corresponding to the "not drawn" line connecting the
data points), two Line2D and one LineCollection.

This patch drops the None out, as it is arguably not a child of the
Container. (It is still possible to access the individual members of
the Container by unpacking it as a tuple.)

Also reformat an overly indented piece of code in the vicinity.

[phobson]

So now it'll return an empty list? Am I reading that right?

Containers are hardly tested. Could we add some to cover this case?

[anntzer]

@phobson Yes, yes, and go ahead :)

[dstansby]

Does this need an api change note?

[anntzer]

Technically the API doesn't change (you can't rely on indexing the results of get_children() anyways as you can get different number of artists (for a given command) depending on how many bars/errorbars/etc. you draw), and I'd guess most people who were using this API either filtered None out or assumed it would never show up.