Optimize Botswana observed holidays

[KJhellico]

Observed holidays calculation method changed. Here are some performance tests.

def calc_hol1():
    for year in range(1966, 2100):
        hols = Botswana(years=year)

def calc_hol2():
    hols = Botswana(years=range(1966, 2100))

t1 = timeit.timeit(calc_hol1, number=100)
t2 = timeit.timeit(calc_hol2, number=100)

Test	Current beta	PR
t1	2.740	2.379
t2	39.242	2.157

[coveralls]

Coverage: 100.0%. Remained the same when pulling 14160da on KJhellico:upd-botswana-observed into 719c267 on dr-prodigy:beta.

[arkid15r]

LGTM