Disambiguate naturaldate return: only append year if more than ~5 months away

[hugovk]

Fixes #26.

Before

Previously, naturaldate appended the year if the input was more than about a year away (>= 365 days).

That led to ambiguity:

>>> import datetime as dt
>>> import humanize
>>> x = dt.datetime.now() - dt.timedelta(3)  # 3 days ago
>>> x
datetime.datetime(2020, 2, 10, 22, 10, 4, 439522)
>>> humanize.naturaldate(x)
'Feb 10'

>>> x = dt.datetime.now() + dt.timedelta(363)  # 363 days forward
>>> x
datetime.datetime(2021, 2, 10, 22, 10, 44, 352492)
>>> humanize.naturaldate(x)
'Feb 10'

And, assuming today is Feb 02 2020, the inputs on the left produced the outputs on the right:

        (dt.date(2019, 2, 2), "Feb 02 2019"),
        (dt.date(2019, 3, 2), "Mar 02"),
        (dt.date(2019, 4, 2), "Apr 02"),
        (dt.date(2019, 5, 2), "May 02"),
        (dt.date(2019, 6, 2), "Jun 02"),
        (dt.date(2019, 7, 2), "Jul 02"),
        (dt.date(2019, 8, 2), "Aug 02"),
        (dt.date(2019, 9, 2), "Sep 02"),
        (dt.date(2019, 10, 2), "Oct 02"),
        (dt.date(2019, 11, 2), "Nov 02"),
        (dt.date(2019, 12, 2), "Dec 02"),
        (dt.date(2020, 1, 2), "Jan 02"),
        (dt.date(2020, 2, 2), "today"),
        (dt.date(2020, 3, 2), "Mar 02"),
        (dt.date(2020, 4, 2), "Apr 02"),
        (dt.date(2020, 5, 2), "May 02"),
        (dt.date(2020, 6, 2), "Jun 02"),
        (dt.date(2020, 7, 2), "Jul 02"),
        (dt.date(2020, 8, 2), "Aug 02"),
        (dt.date(2020, 9, 2), "Sep 02"),
        (dt.date(2020, 10, 2), "Oct 02"),
        (dt.date(2020, 11, 2), "Nov 02"),
        (dt.date(2020, 12, 2), "Dec 02"),
        (dt.date(2021, 1, 2), "Jan 02"),
        (dt.date(2021, 2, 2), "Feb 02 2021"),

After

Only append the year if the input is more than about 5 months away (>= 5 * 30.5 days).

>>> import datetime as dt
>>> import humanize
>>> x = dt.datetime.now() - dt.timedelta(3)  # 3 days ago
>>> x
datetime.datetime(2020, 2, 10, 22, 13, 56, 74705)
>>> humanize.naturaldate(x)
'Feb 10'

>>> x = dt.datetime.now() + dt.timedelta(363)  # 363 days forward
>>> x
datetime.datetime(2021, 2, 10, 22, 14, 5, 816049)
>>> humanize.naturaldate(x)
'Feb 10 2021'

        (dt.date(2019, 2, 2), "Feb 02 2019"),
        (dt.date(2019, 3, 2), "Mar 02 2019"),
        (dt.date(2019, 4, 2), "Apr 02 2019"),
        (dt.date(2019, 5, 2), "May 02 2019"),
        (dt.date(2019, 6, 2), "Jun 02 2019"),
        (dt.date(2019, 7, 2), "Jul 02 2019"),
        (dt.date(2019, 8, 2), "Aug 02 2019"),
        (dt.date(2019, 9, 2), "Sep 02 2019"),
        (dt.date(2019, 10, 2), "Oct 02"),
        (dt.date(2019, 11, 2), "Nov 02"),
        (dt.date(2019, 12, 2), "Dec 02"),
        (dt.date(2020, 1, 2), "Jan 02"),
        (dt.date(2020, 2, 2), "today"),
        (dt.date(2020, 3, 2), "Mar 02"),
        (dt.date(2020, 4, 2), "Apr 02"),
        (dt.date(2020, 5, 2), "May 02"),
        (dt.date(2020, 6, 2), "Jun 02"),
        (dt.date(2020, 7, 2), "Jul 02"),
        (dt.date(2020, 8, 2), "Aug 02 2020"),
        (dt.date(2020, 9, 2), "Sep 02 2020"),
        (dt.date(2020, 10, 2), "Oct 02 2020"),
        (dt.date(2020, 11, 2), "Nov 02 2020"),
        (dt.date(2020, 12, 2), "Dec 02 2020"),
        (dt.date(2021, 1, 2), "Jan 02 2021"),
        (dt.date(2021, 2, 2), "Feb 02 2021"),