Rename flatten array to flatten

[Insti]

@treyhunner wrote

Opened from a discussion in exercism/python#374.

I think this exercise should be renamed to use more generic terminology. flatten is sufficiently generic. Arrays in Python are different than lists and are not frequently used.

There is further discussion about what is required in this closed pull request: #412

If you wish to work on this, I suggest reading that issue and posting a plan of action to this issue for review before proceeding with the pull request(s) required to implement the changes.

[petertseng]

For those considering performing this rename, there could be some useful information in exercism/discussions#12