Enhance and move ISO-8601 parser to coding.times

[kmuehlbauer]

This enhances the ISO-8601 parser to check for signed years (eg, -0300 or +0010) as well as 5 digit years (eg. -10000, +20000, 10000). The relevant parsing code is moved to coding.times (to prevent circular import) as these functionality will be used in #9618 within coding.times.

• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[kmuehlbauer]

@dcherian Thanks for the review, I've addressed according your comments and suggestions.

[spencerkclark]

Thanks @kmuehlbauer—this looks nice. I guess so long as we accept a flexible number of digits in the year it will be hard to extend beyond five-digit years, which is totally fine for now.

Per @dcherian's TODO, I tried my hand at adding a hypothesis strategy for generating cftime objects in kmuehlbauer#2.

[headtr1ck]

Let's take the opportunity and add some typing.

But looks good, thanks.

[kmuehlbauer]

Let's take the opportunity and add some typing.

Thanks @headtr1ck for the review and typing addendum.

[kmuehlbauer]

Per @dcherian's TODO, I tried my hand at adding a hypothesis strategy for generating cftime objects in kmuehlbauer#2.

Thanks @spencerkclark for reviewing and the additions!

[spencerkclark]

Thanks @kmuehlbauer! Though it passes CI, I'm happy to iterate further on the hypothesis portion in this or a follow-up PR if needed. It's fairly low-stakes / orthogonal to the main contribution.

[dcherian]

@spencerkclark very nice!

I think the idiomatic way is to use composite but we can do that later.