Improve handling of imaginary datetimes

[systemcatch]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [x] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [x] ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

closes #72

Pendulum example is now fixed.

(arrow) chris@ThinkPad:~/arrow$ python
Python 3.8.3 (default, Jul  7 2020, 18:57:36) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import arrow
>>> just_before = arrow.get(2013, 3, 31, 1, 55, "Europe/Paris")
>>> just_before = arrow.get(2013, 3, 31, 1, 55, tzinfo= "Europe/Paris")
>>> just_before
<Arrow [2013-03-31T01:55:00+01:00]>
>>> just_before.shift(minutes=+10)
<Arrow [2013-03-31T03:05:00+02:00]>

Range duplication no longer occurs.

>>> before = arrow.get('2018-03-10 23:00:00', 'YYYY-MM-DD HH:mm:ss', tzinfo='US/Pacific')
>>> after = arrow.get('2018-03-11 04:00:00', 'YYYY-MM-DD HH:mm:ss', tzinfo='US/Pacific')
>>> result=[ (t, t.to('utc')) for t in arrow.Arrow.range('hour', before, after) ]
>>> for r in result:
...     print(r)
... 
(<Arrow [2018-03-10T23:00:00-08:00]>, <Arrow [2018-03-11T07:00:00+00:00]>)
(<Arrow [2018-03-11T00:00:00-08:00]>, <Arrow [2018-03-11T08:00:00+00:00]>)
(<Arrow [2018-03-11T01:00:00-08:00]>, <Arrow [2018-03-11T09:00:00+00:00]>)
(<Arrow [2018-03-11T03:00:00-07:00]>, <Arrow [2018-03-11T10:00:00+00:00]>)
(<Arrow [2018-03-11T04:00:00-07:00]>, <Arrow [2018-03-11T11:00:00+00:00]>)

I was also considering wrapping dateutil's resolve_imaginary function as an utility method for our users.

[codecov[bot]]

Codecov Report

Merging #845 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #845   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines         1786      1791    +5     
  Branches       307       308    +1     
=========================================
+ Hits          1786      1791    +5     

Impacted Files	Coverage Δ
arrow/arrow.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 80aa80c...7d225c1. Read the comment docs.

[systemcatch]

First pass looks solid. Is this seriously all the places where we need to cover imaginary dates/times?

I think so, by fixing shift we can use it in range and span which then propagate to other methods.

Does this address https://github.com/arrow-py/arrow/issues/686 as well? If so, we should add a regression test for it.

No and looking at it I don't think that issue can be fixed in arrow. Those comparison results are baked into datetime itself.

[systemcatch]

@pganssle any thoughts to add here?

[systemcatch]

Should we also resolve imaginary datetimes if they are constructed via the arrow.get or arrow.Arrow?

I tend to think not as datetime lets you construct imaginary instances directly. Users might have a reason for needing them and with the new imaginary property it's easy to check it

[systemcatch]

Hey @jadchaar I've made the change you suggested, let me know if you're happy and I will merge this in.

[jadchaar]

Looks great! Ship it!