Optimize Ukraine observed holidays

[KJhellico]

Performance tests:

country = "UA"
START_YEAR = 1917
END_YEAR = 2099

def calc_hol1():
    for year in range(START_YEAR, END_YEAR + 1):
        hols = holidays.country_holidays(country, years=year)

def calc_hol2():
    hols = holidays.country_holidays(country, years=range(START_YEAR, END_YEAR + 1))

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

Test	Current beta	PR
t1	2.290	2.219
t2	19.376	1.925

[coveralls]

Coverage: 100.0%. Remained the same when pulling 1e2414b on KJhellico:upd-ukraine-observed into 719c267 on dr-prodigy:beta.

[arkid15r]

LGTM