concatenate - serial comma incorrectly used with two items

[jsamarziya]

Per Wikipedia, a serial comma is

a comma placed immediately after the penultimate term... in a series of three or more terms

We would therefore expect that a serial comma would not be placed by concatenate() when given a sequence of two items, but this is not the case.

Expected:

>>> humanfriendly.text.concatenate(items=["foo", "bar"], serial_comma=True)
'foo and bar'

Actual:

>>> humanfriendly.text.concatenate(items=["foo", "bar"], serial_comma=True)
'foo, and bar'

Workaround:

>>> items = ["foo", "bar"]
>>> humanfriendly.text.concatenate(items=items, serial_comma=len(items) > 2)
'foo and bar'