bpo-34158: Documentation UTC offset update

[christopheNan]

Since changes in the UTC offset that allows sub-minute offsets, the documentation needs update:

• "%z" format code documentation update
  Karthikeyan Singaravelan commented on bugs.python.org:
  Added as part of 018d353 and a fix regarding duplicate words for that part was added at bac2d5b.
  Relevant format string at https://github.com/python/cpython/pull/2896/files#diff-25e2d173c84057d069b7890450714eddR214.
  Relevant test case with 6-digit string for microsecond : https://github.com/python/cpython/pull/2896/files#diff-acc40bec51c7de832de3361db3edae52R309.
  Table at https://docs.python.org/3.7/library/datetime.html#strftime-and-strptime-behavior could also be updated with microseconds being optional in the second column
  %z | UTC offset in the form ±HHMM[SS] (empty string if the object is naive). | (empty), +0000, -0400, +1030

• isoformat documentation update
  According to me, needs confirmation:
  Relevant format string at https://github.com/python/cpython/pull/4699/files#diff-25e2d173c84057d069b7890450714eddR176
  Relevant test case at https://github.com/python/cpython/pull/4699/files#diff-acc40bec51c7de832de3361db3edae52R1734

https://bugs.python.org/issue34158

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[christopheNan]

CLA signed.

[vadmium]

Would be more consistent to use mmmmmm rather than uuuuuu.

Since you are introducing square brackets, you could eliminate the second form and just write [.mmmmmm] for both time and offset parts.

[christopheNan]

Well, I used 'u' from the '%z' format code.
And it sounds better to me ('u' for 'micro') than 'm' which usually stands for 'milli'

[pganssle]

Hm, we are wildly inconsistent in this document.

From the fromisoformat documentation (which I wrote 😳):

Specifically, this function supports strings in the format(s)
YYYY-MM-DD[*HH[:MM[:SS[.mmm[mmm]]]][+HH:MM[:SS[.ffffff]]]],
where * can match any single character.

That uses both fff and mmm in the same format string.

moment.js uses SS for fractional seconds, but I find that very confusing.

I think that f is most consistent with the %f notation in str{f,p}time, and avoids the question of whether it should be [.mmm[.uuu]].

[miss-islington]

Thanks @christopheNan for the PR, and @abalkin for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-9732 is a backport of this pull request to the 3.7 branch.