Allow getitem from a cycler

It would be useful to add __getitem__ functionality to cycler.

The use case is a "crowded" for loop where you want to get the cycler style without using a loop variable. Something like:

data_cycler = cycler('alpha', (0.2, 0.4, 0.6)) + cycler('color', ('r', 'g', 'b'))
fit_cycler = cycler('color', ('k', 'm', 'orange'))

for i, (data, fit) in enumerate(zip(data_list, fit_list)):
    ...
    plt.plot('x', 'y', data=data, data_cycler[i])
    plt.plot('x', 'y', data=data, fit_cycler[i])
    ...

In this case the index i is computed anyway, so using it for the cyclers would be handy, instead of adding the cycler inside zip().

Ideally mycycler[i] should return the item with index i % len(mycycler).

I haven't looked at the code yet. Would this be possible? Would a PR be welcome?

首先阅读 cycler 的实现及其现有测试，重点关注循环的迭代方式以及其长度的公开方式。添加具有所需取模行为的索引访问，并更新测试以展示 mycycler[i] 返回位于 i % len(mycycler) 的项。