Refactor days offset calculation: replace relativedelta with timedelta

[arkid15r]

Resolves #891

This PR contains business logic only changes. See #901 for tests changes.

My simple benchmark results for the following code:

def create_country_holidays():
    for country in list_supported_countries():
        country_holidays(country, years=range(2020, 2031))

• the relativedelta:

timeit.timeit(create_country_holidays, number=1000)
339.802383083

• the timedelta:

timeit.timeit(create_country_holidays, number=1000)
313.579699042

[coveralls]

Coverage: 100.0%. Remained the same when pulling f49aa89 on arkid15r:timedelta-migration into c8b2d0c on dr-prodigy:beta.

[KJhellico]

Maybe import timedelta as td? Then lines length will be same.

[arkid15r]

Actually, it was my original intent to do it that way. I'm not sure why I didn't stick to the plan. Thank for pointing out, @KJhellico!

It's great to have you back! It felt somewhat empty without your reviews here. I really appreciate them!