Fix humanize reporting "a year ago" for ~31-day deltas in same month

[bysiber]

Summary

Fix a gap in the humanize() if/elif chain that causes ~31-day deltas within the same calendar month to report "a year ago" instead of "a month ago".

Problem

In the auto-granularity branch of humanize(), there's a gap between the "weeks" and "months" conditions:

elif calendar_months < 1 and diff < self._SECS_PER_MONTH:
    # "X weeks ago"

elif calendar_months >= 1 and diff < self._SECS_PER_YEAR:
    # "X months ago"

elif diff < self._SECS_PER_YEAR * 2:
    # "a year ago"

When diff >= _SECS_PER_MONTH (30.5 days) and calendar_months < 1 (both dates in the same calendar month), neither the "weeks" nor the "months" condition is satisfied:

• "weeks" requires diff < _SECS_PER_MONTH → False
• "months" requires calendar_months >= 1 → False

Execution falls through to the "a year ago" branch.

Reproduction

import arrow

start = arrow.Arrow(2023, 1, 1)
end = arrow.Arrow(2023, 1, 31, 14, 0, 0)  # 30.5+ days later, same month

start.humanize(end)  # Returns "a year ago" instead of "a month ago"

This affects all 7 months with 31 days (January, March, May, July, August, October, December) when comparing timestamps near the boundaries of the same month.

Fix

Add an explicit branch for this case: when calendar_months < 1 but diff >= _SECS_PER_MONTH, report the delta as "a month".

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.91%. Comparing base (b423717) to head (9551f97).

Files with missing lines	Patch %	Lines
arrow/arrow.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##            master    #1247      +/-   ##
===========================================
- Coverage   100.00%   99.91%   -0.09%     
===========================================
  Files           10       10              
  Lines         2315     2317       +2     
  Branches       358      359       +1     
===========================================
  Hits          2315     2315              
- Misses           0        1       +1     
- Partials         0        1       +1     

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[krisfremen]

@bysiber please add tests, otherwise this looks to be good

[bysiber]

Added a test, it checks the case where you have a ~31-day delta between Jan 15 and Feb 15, where calendar_months can round to 0 but the actual elapsed time exceeds one month. Without the fix, this case fell through to the wrong branch.