Makes toyplot work with python 3.10 due to an API change in collections

[StuntsPT]

Using toyplot with python 3.10 was causing the following exception:

File "/home/francisco/.local/lib/python3.10/site-packages/toyplot/color.py", line 200, in broadcast
    if isinstance(colors, collections.Sequence):
AttributeError: module 'collections' has no attribute 'Sequence'

Investigating the issue led me to realize this was due to a change in the collections module.
The class Sequence was moved from collections.Sequence to collections.abc.Sequence.
This PR fixes the issue by calling the Sequence class from the "new" module.
I did not test it, but according to the documentation, this should be compatible with python 3.3 and up.

Best,

Francisco

[coveralls]

Coverage remained the same at 94.969% when pulling 857778a on StuntsPT:main into c294153 on sandialabs:main.

[tshead2]

@StuntsPT - great job tracking this down. Many thanks!

Tim

[StuntsPT]

Happy to help!