Optimize Burundi observed holidays

[KJhellico]

Performance tests:

def calc_hol1():
    for year in range(1962, 2100):
        hols = Burundi(years=year)

def calc_hol2():
    hols = Burundi(years=range(1962, 2100))

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

Test	Current beta	PR
t1	3.114	3.117
t2	6.797	2.920

[coveralls]

Coverage: ?%. Remained the same when pulling c78afe1 on KJhellico:upd-burundi-observed into 719c267 on dr-prodigy:beta.

[coveralls]

Coverage: 100.0%. Remained the same when pulling c78afe1 on KJhellico:upd-burundi-observed into 719c267 on dr-prodigy:beta.

[arkid15r]

LGTM

// You can skip the performance tests part as we both agreed on these changes earlier